Fix a valgrind unit test failure (#13137)

Summary:
fix the valgrind failure from https://github.com/facebook/rocksdb/actions/runs/11813904728/job/32911902535?fbclid=IwZXh0bgNhZW0CMTEAAR2GJs1U6mNwNv3zwPzU8rpCmBHqfStV3dupj2o_-686RneLKXADaSZH5-U_aem_ADUQy7bzknoseVpjrOc5SQ
```
[ RUN      ] WBWIMemTableTest.ReadFromWBWIMemtable
==1150870== Conditional jump or move depends on uninitialised value(s)
==1150870==    at 0x50FE67A: rocksdb::WBWIMemTable::Get(rocksdb::LookupKey const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, rocksdb::PinnableWideColumns*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, rocksdb::Status*, rocksdb::MergeContext*, unsigned long*, unsigned long*, rocksdb::ReadOptions const&, bool, rocksdb::ReadCallback*, bool*, bool) (wbwi_memtable.cc:60)
==1150870==    by 0x50FF92A: rocksdb::WBWIMemTable::MultiGet(rocksdb::ReadOptions const&, rocksdb::MultiGetContext::Range*, rocksdb::ReadCallback*, bool) (wbwi_memtable.cc:120)
==1150870==    by 0x1879EF: rocksdb::WBWIMemTableTest_ReadFromWBWIMemtable_Test::TestBody() (write_batch_with_index_test.cc:3580)
```

Pull Request resolved: #13137

Test Plan: `valgrind ./write_batch_with_index_test --gtest_filter="*ReadFromWBWIMemtable*"`

Reviewed By: ltamasi

Differential Revision: D65892657

Pulled By: cbi42

fbshipit-source-id: 0b44a5a06b8cc64173ad36966339877e2f508d52

memtable/wbwi_memtable.cc

@@ -115,7 +115,7 @@ void WBWIMemTable::MultiGet(const ReadOptions& read_options,
   assert(immutable_memtable);
   // TODO: reuse the InternalIterator created in Get().
   for (auto iter = range->begin(); iter != range->end(); ++iter) {
-    SequenceNumber dummy_seq;
+    SequenceNumber dummy_seq = 0;
     bool found_final_value =
         Get(*iter->lkey, iter->value ? iter->value->GetSelf() : nullptr,
             iter->columns, iter->timestamp, iter->s, &(iter->merge_context),