Replace unsafe string construction with bufferlist::length() to avoid
reading beyond buffer boundaries.

In commit 92ccbff, we introduced a bug when checking if a digest was
empty by constructing a std::string from bufferlist:

```
std::string(digest.second.c_str()).empty()
```

This is unsafe because bufferlist data is not guaranteed to be null-
terminated. The std::string constructor searches for a null terminator
and may read beyond the bufferlist's allocated memory, causing a
heap-buffer-overflow detected by AddressSanitizer:

```
==66092==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7e0c65215004 at pc 0x7fbc6e27c597 bp 0x7ffe29fb6100 sp 0x7ffe29fb58b8
READ of size 5 at 0x7e0c65215004 thread T0
    #0 0x7fbc6e27c596 in strlen /usr/src/debug/gcc/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:425
    #1 0x562c75fad91a in std::char_traits<char>::length(char const*) /usr/include/c++/15.2.1/bits/char_traits.h:393
    #2 0x562c75fb4222 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string<std::allocator<char> >(char const*, std::allocator<char> const&) /usr/include/c++/15.2.1/bits/b
asic_string.h:713
    #3 0x562c761b81ae in operator() /home/kefu/dev/ceph/src/osd/scrubber/scrub_backend.cc:1300
    #4 0x562c761d7d53 in operator()<mini_flat_map<shard_id_t, ceph::buffer::v15_2_0::list, signed char>::_iterator<false> > /usr/include/c++/15.2.1/bits/predefined_ops.h:318
    #5 0x562c761d789c in __find_if<mini_flat_map<shard_id_t, ceph::buffer::v15_2_0::list, signed char>::_iterator<false>, __gnu_cxx::__ops::_Iter_pred<ScrubBackend::match_in_shards(const hobject_t&, auth_selection_
t&, inconsistent_obj_wrapper&, std::stringstream&)::<lambda(const std::pair<const shard_id_t, ceph::buffer::v15_2_0::list&>&)> > > /usr/include/c++/15.2.1/bits/stl_algobase.h:2095
    #6 0x562c761d72b2 in find_if<mini_flat_map<shard_id_t, ceph::buffer::v15_2_0::list, signed char>::_iterator<false>, ScrubBackend::match_in_shards(const hobject_t&, auth_selection_t&, inconsistent_obj_wrapper&,
std::stringstream&)::<lambda(const std::pair<const shard_id_t, ceph::buffer::v15_2_0::list&>&)> > /usr/include/c++/15.2.1/bits/stl_algo.h:3921
    #7 0x562c761d5f6f in none_of<mini_flat_map<shard_id_t, ceph::buffer::v15_2_0::list, signed char>::_iterator<false>, ScrubBackend::match_in_shards(const hobject_t&, auth_selection_t&, inconsistent_obj_wrapper&,
std::stringstream&)::<lambda(const std::pair<const shard_id_t, ceph::buffer::v15_2_0::list&>&)> > /usr/include/c++/15.2.1/bits/stl_algo.h:431
    #8 0x562c761d4a50 in any_of<mini_flat_map<shard_id_t, ceph::buffer::v15_2_0::list, signed char>::_iterator<false>, ScrubBackend::match_in_shards(const hobject_t&, auth_selection_t&, inconsistent_obj_wrapper&, s
td::stringstream&)::<lambda(const std::pair<const shard_id_t, ceph::buffer::v15_2_0::list&>&)> > /usr/include/c++/15.2.1/bits/stl_algo.h:450
    #9 0x562c761bb84b in ScrubBackend::match_in_shards(hobject_t const&, auth_selection_t&, inconsistent_obj_wrapper&, std::__cxx11::basic_stringstream<char, std::char_traits<char>, std::allocator<char> >&) /home/k
efu/dev/ceph/src/osd/scrubber/scrub_backend.cc:1297
    #10 0x562c761b4282 in ScrubBackend::compare_obj_in_maps[abi:cxx11](hobject_t const&) /home/kefu/dev/ceph/src/osd/scrubber/scrub_backend.cc:941
    #11 0x562c761d44af in operator()<hobject_t> /home/kefu/dev/ceph/src/osd/scrubber/scrub_backend.cc:887
    #12 0x562c761d4836 in for_each<std::_Rb_tree_const_iterator<hobject_t>, ScrubBackend::compare_smaps()::<lambda(const auto:422&)> > /usr/include/c++/15.2.1/bits/stl_algo.h:3798
    ceph#13 0x562c761b3259 in ScrubBackend::compare_smaps() /home/kefu/dev/ceph/src/osd/scrubber/scrub_backend.cc:884
    ceph#14 0x562c761a478d in ScrubBackend::update_authoritative() /home/kefu/dev/ceph/src/osd/scrubber/scrub_backend.cc:315`
```

Fix by using bufferlist::length() which tells if the given buffer is
empty instead of converting the buffer content to a string.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>