`max_domains * sizeof(uint64_t)` can overflow, and then
the for loop below would write beyond the end of `current_positions`.

All other bounds checks can be done in `read_poll`, but this one must be
done here to avoid potential memory corruption.

Add a testcase which takes a valid ring, and corrupts some fields in it.
For now it only corrupt the max_domains fields, but this will be
extended in followup commits.

Bug found by ASAN, and also reproducible with the included testcase.

Signed-off-by: Edwin Török <edwin.torok@cloud.com>

msg_length zero could cause us to loop infinitely.

All messages have at least a header and a timestamp,
except EV_INTERNAL (0), which is a padding event.

Extend the testcase by corrupting message lengths.

This also avoids some memory sanitizer warnings.

Rejecting all msg_length < 2 would cause a failure in
test_dropped_events.ml, because padding events would be incorrectly
rejected.

Suggested-by: Miod Vallat <miod@tarides.com>
Signed-off-by: Edwin Török <edwin.torok@cloud.com>

Protect against metadata header getting corrupted, and make a copy at
cursor open time.

Detect corrupted runtime events rings, check that:

* metadata, data and custom offsets are within file bounds
* record end is within file bounds
* masked offset is within bounds
* memcpy is within bounds
* ring_size_elements cannot be 0, because then the mask computation is
  not valid anymore

The bounds checks are performed as each entry is processed, to allow
processing the maximum number of uncorrupted entries before raising
exceptions.

Found by ASAN, and (mostly) reproducible with the included testcase.

Co-authored-by: Miod Vallat <118974489+dustanddreams@users.noreply.github.com>
Signed-off-by: Edwin Török <edwin.torok@cloud.com>

Found by -fsanitize=memory -fsanitize-memory-track-origins:
```
> ==102752==WARNING: MemorySanitizer: use-of-uninitialized-value
>     #0 0x7f2ba7fb4ea4 in caml_runtime_events_read_poll /var/home/edwin/git/ocaml/otherlibs/runtime_events/runtime_events_consumer.c:496:18
>     #1 0x7f2ba7fbc016 in caml_ml_runtime_events_read_poll /var/home/edwin/git/ocaml/otherlibs/runtime_events/runtime_events_consumer.c:1207:9
>     ocaml#2 0x59ba5c in caml_interprete /var/home/edwin/git/ocaml/runtime/interp.c:1058:14
>     ocaml#3 0x5a9220 in caml_main /var/home/edwin/git/ocaml/runtime/startup_byt.c:575:9
>     ocaml#4 0x540d6b in main /var/home/edwin/git/ocaml/runtime/main.c:37:3
>     ocaml#5 0x7f2ba8120087 in __libc_start_call_main (/lib64/libc.so.6+0x2a087) (BuildId: 8f53abaad945a669f2bdcd25f471d80e077568ef)
>     ocaml#6 0x7f2ba812014a in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x2a14a) (BuildId: 8f53abaad945a669f2bdcd25f471d80e077568ef)
>     ocaml#7 0x441804 in _start (/var/home/edwin/git/ocaml/runtime/ocamlrun+0x441804) (BuildId: 617637580ee48eff08a2bce790e1667ad09f3b69)
>
>   Uninitialized value was stored to memory at
>     #0 0x7f2ba7fb4e9d in caml_runtime_events_read_poll /var/home/edwin/git/ocaml/otherlibs/runtime_events/runtime_events_consumer.c:497:69
>     #1 0x7f2ba7fbc016 in caml_ml_runtime_events_read_poll /var/home/edwin/git/ocaml/otherlibs/runtime_events/runtime_events_consumer.c:1207:9
>     ocaml#2 0x59ba5c in caml_interprete /var/home/edwin/git/ocaml/runtime/interp.c:1058:14
>     ocaml#3 0x5a9220 in caml_main /var/home/edwin/git/ocaml/runtime/startup_byt.c:575:9
>     ocaml#4 0x540d6b in main /var/home/edwin/git/ocaml/runtime/main.c:37:3
>     ocaml#5 0x7f2ba8120087 in __libc_start_call_main (/lib64/libc.so.6+0x2a087) (BuildId: 8f53abaad945a669f2bdcd25f471d80e077568ef)
>     ocaml#6 0x7f2ba812014a in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x2a14a) (BuildId: 8f53abaad945a669f2bdcd25f471d80e077568ef)
>     ocaml#7 0x441804 in _start (/var/home/edwin/git/ocaml/runtime/ocamlrun+0x441804) (BuildId: 617637580ee48eff08a2bce790e1667ad09f3b69)
>
>   Uninitialized value was created by an allocation of 'buf' in the stack frame
>     #0 0x7f2ba7fb3dbc in caml_runtime_events_read_poll /var/home/edwin/git/ocaml/otherlibs/runtime_events/runtime_events_consumer.c:402:7
>
```

This is in fact an EV_LIFECYCLE with EV_RING_STOP, which has 0
additional data, and thus msg_length 2:
```
runtime/runtime_events.c:      EV_RUNTIME, (ev_message_type){.runtime=EV_LIFECYCLE}, EV_RING_STOP, 0,
```

Attempting to read from `buf[2]` would read uninitialized data.

Signed-off-by: Edwin Török <edwin.torok@cloud.com>

It is not enough to check for bytes = elements * 8,
because elements * 8 could overflow.

Bug found by AFL (resulting in a hang).

Signed-off-by: Edwin Török <edwin.torok@cloud.com>

Signed-off-by: Edwin Török <edwin.torok@cloud.com>