…y-io#2257)

**Current state**
During `hashtableScanDefrag`, rehashing is paused to prevent entries
from moving, but the scan callback can still delete entries which
triggers `hashtableShrinkIfNeeded`. For example, the
`expireScanCallback` can delete expired entries.

**Issue**
This can cause the table to be resized and the old memory to be freed
while the scan is still accessing it, resulting in the following memory
access violation:

```
[err]: Sanitizer error: =================================================================
==46774==ERROR: AddressSanitizer: heap-use-after-free on address 0x611000003100 at pc 0x0000004704d3 bp 0x7fffcb062000 sp 0x7fffcb061ff0
READ of size 1 at 0x611000003100 thread T0
    #0 0x4704d2 in isPositionFilled /home/gusakovy/Projects/valkey/src/hashtable.c:422
    #1 0x478b45 in hashtableScanDefrag /home/gusakovy/Projects/valkey/src/hashtable.c:1768
    #2 0x4789c2 in hashtableScan /home/gusakovy/Projects/valkey/src/hashtable.c:1729
    #3 0x47e3ca in kvstoreScan /home/gusakovy/Projects/valkey/src/kvstore.c:402
    #4 0x6d9040 in activeExpireCycle /home/gusakovy/Projects/valkey/src/expire.c:297
    #5 0x4859d2 in databasesCron /home/gusakovy/Projects/valkey/src/server.c:1269
    #6 0x486e92 in serverCron /home/gusakovy/Projects/valkey/src/server.c:1577
    #7 0x4637dd in processTimeEvents /home/gusakovy/Projects/valkey/src/ae.c:370
    #8 0x4643e3 in aeProcessEvents /home/gusakovy/Projects/valkey/src/ae.c:513
    #9 0x4647ea in aeMain /home/gusakovy/Projects/valkey/src/ae.c:543
    #10 0x4a61fc in main /home/gusakovy/Projects/valkey/src/server.c:7291
    #11 0x7f471957c139 in __libc_start_main (/lib64/libc.so.6+0x21139)
    #12 0x452e39 in _start (/local/home/gusakovy/Projects/valkey/src/valkey-server+0x452e39)

0x611000003100 is located 0 bytes inside of 256-byte region [0x611000003100,0x611000003200)
freed by thread T0 here:
    #0 0x7f471a34a1e5 in __interceptor_free (/lib64/libasan.so.4+0xd81e5)
    #1 0x4aefbc in zfree_internal /home/gusakovy/Projects/valkey/src/zmalloc.c:400
    #2 0x4aeff5 in valkey_free /home/gusakovy/Projects/valkey/src/zmalloc.c:415
    #3 0x4707d2 in rehashingCompleted /home/gusakovy/Projects/valkey/src/hashtable.c:456
    #4 0x471b5b in resize /home/gusakovy/Projects/valkey/src/hashtable.c:656
    #5 0x475bff in hashtableShrinkIfNeeded /home/gusakovy/Projects/valkey/src/hashtable.c:1272
    #6 0x47704b in hashtablePop /home/gusakovy/Projects/valkey/src/hashtable.c:1448
    #7 0x47716f in hashtableDelete /home/gusakovy/Projects/valkey/src/hashtable.c:1459
    #8 0x480038 in kvstoreHashtableDelete /home/gusakovy/Projects/valkey/src/kvstore.c:847
    #9 0x50c12c in dbGenericDeleteWithDictIndex /home/gusakovy/Projects/valkey/src/db.c:490
    #10 0x515f28 in deleteExpiredKeyAndPropagateWithDictIndex /home/gusakovy/Projects/valkey/src/db.c:1831
    #11 0x516103 in deleteExpiredKeyAndPropagate /home/gusakovy/Projects/valkey/src/db.c:1844
    #12 0x6d8642 in activeExpireCycleTryExpire /home/gusakovy/Projects/valkey/src/expire.c:70
    #13 0x6d8706 in expireScanCallback /home/gusakovy/Projects/valkey/src/expire.c:139
    #14 0x478bd8 in hashtableScanDefrag /home/gusakovy/Projects/valkey/src/hashtable.c:1770
    #15 0x4789c2 in hashtableScan /home/gusakovy/Projects/valkey/src/hashtable.c:1729
    #16 0x47e3ca in kvstoreScan /home/gusakovy/Projects/valkey/src/kvstore.c:402
    #17 0x6d9040 in activeExpireCycle /home/gusakovy/Projects/valkey/src/expire.c:297
    #18 0x4859d2 in databasesCron /home/gusakovy/Projects/valkey/src/server.c:1269
    #19 0x486e92 in serverCron /home/gusakovy/Projects/valkey/src/server.c:1577
    #20 0x4637dd in processTimeEvents /home/gusakovy/Projects/valkey/src/ae.c:370
    #21 0x4643e3 in aeProcessEvents /home/gusakovy/Projects/valkey/src/ae.c:513
    #22 0x4647ea in aeMain /home/gusakovy/Projects/valkey/src/ae.c:543
    #23 0x4a61fc in main /home/gusakovy/Projects/valkey/src/server.c:7291
    #24 0x7f471957c139 in __libc_start_main (/lib64/libc.so.6+0x21139)

previously allocated by thread T0 here:
    #0 0x7f471a34a753 in __interceptor_calloc (/lib64/libasan.so.4+0xd8753)
    #1 0x4ae48c in ztrycalloc_usable_internal /home/gusakovy/Projects/valkey/src/zmalloc.c:214
    #2 0x4ae757 in valkey_calloc /home/gusakovy/Projects/valkey/src/zmalloc.c:257
    #3 0x4718fc in resize /home/gusakovy/Projects/valkey/src/hashtable.c:645
    #4 0x475bff in hashtableShrinkIfNeeded /home/gusakovy/Projects/valkey/src/hashtable.c:1272
    #5 0x47704b in hashtablePop /home/gusakovy/Projects/valkey/src/hashtable.c:1448
    #6 0x47716f in hashtableDelete /home/gusakovy/Projects/valkey/src/hashtable.c:1459
    #7 0x480038 in kvstoreHashtableDelete /home/gusakovy/Projects/valkey/src/kvstore.c:847
    #8 0x50c12c in dbGenericDeleteWithDictIndex /home/gusakovy/Projects/valkey/src/db.c:490
    #9 0x515f28 in deleteExpiredKeyAndPropagateWithDictIndex /home/gusakovy/Projects/valkey/src/db.c:1831
    #10 0x516103 in deleteExpiredKeyAndPropagate /home/gusakovy/Projects/valkey/src/db.c:1844
    #11 0x6d8642 in activeExpireCycleTryExpire /home/gusakovy/Projects/valkey/src/expire.c:70
    #12 0x6d8706 in expireScanCallback /home/gusakovy/Projects/valkey/src/expire.c:139
    #13 0x478bd8 in hashtableScanDefrag /home/gusakovy/Projects/valkey/src/hashtable.c:1770
    #14 0x4789c2 in hashtableScan /home/gusakovy/Projects/valkey/src/hashtable.c:1729
    #15 0x47e3ca in kvstoreScan /home/gusakovy/Projects/valkey/src/kvstore.c:402
    #16 0x6d9040 in activeExpireCycle /home/gusakovy/Projects/valkey/src/expire.c:297
    #17 0x4859d2 in databasesCron /home/gusakovy/Projects/valkey/src/server.c:1269
    #18 0x486e92 in serverCron /home/gusakovy/Projects/valkey/src/server.c:1577
    #19 0x4637dd in processTimeEvents /home/gusakovy/Projects/valkey/src/ae.c:370
    #20 0x4643e3 in aeProcessEvents /home/gusakovy/Projects/valkey/src/ae.c:513
    #21 0x4647ea in aeMain /home/gusakovy/Projects/valkey/src/ae.c:543
    #22 0x4a61fc in main /home/gusakovy/Projects/valkey/src/server.c:7291
    #23 0x7f471957c139 in __libc_start_main (/lib64/libc.so.6+0x21139)

SUMMARY: AddressSanitizer: heap-use-after-free /home/gusakovy/Projects/valkey/src/hashtable.c:422 in isPositionFilled
Shadow bytes around the buggy address:
  0x0c227fff85d0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c227fff85e0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c227fff85f0: fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd
  0x0c227fff8600: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c227fff8610: fd fd fd fd fd fd fd fd fa fa fa fa fa fa fa fa
=>0x0c227fff8620:[fd]fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c227fff8630: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c227fff8640: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c227fff8650: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c227fff8660: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c227fff8670: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==46774==ABORTING
```


**Solution**
Suggested solution is to also pause auto shrinking during
`hashtableScanDefrag`. I noticed that there was already a
`hashtablePauseAutoShrink` method and `pause_auto_shrink` counter, but
it wasn't actually used in `hashtableShrinkIfNeeded` so I fixed that.

**Testing**
I created a simple tcl test that (most of the times) triggers this
error, but it's a little clunky so I didn't add it as part of the PR:

```
start_server {tags {"expire hashtable defrag"}} {
    test {hashtable scan defrag on expiry} {

        r config set hz 100

        set num_keys 20
        for {set i 0} {$i < $num_keys} {incr i} {
            r set "key_$i" "value_$i"
        }

        for {set j 0} {$j < 50} {incr j} {
            set expire_keys 100
            for {set i 0} {$i < $expire_keys} {incr i} {
                # Short expiry time to ensure they expire quickly
                r psetex "expire_key_${i}_${j}" 100 "expire_value_${i}_${j}"
            }

            # Verify keys are set
            set initial_size [r dbsize]
            assert_equal $initial_size [expr $num_keys + $expire_keys]
            
            after 150
            for {set i 0} {$i < 10} {incr i} {
                r get "expire_key_${i}_${j}"
                after 10
            }
        }

        set remaining_keys [r dbsize]
        assert_equal $remaining_keys $num_keys

        # Verify server is still responsive
        assert_equal [r ping] {PONG}
    } {}
}
```
Compiling with ASAN using `make noopt SANITIZER=address valkey-server`
and running the test causes error above. Applying the fix resolves the
issue.

Signed-off-by: Yakov Gusakov <yaakov0015@gmail.com>