Fix -Werror=maybe-uninitialized in db_stress_tool (#9265)

Summary:
Context/Summary:
Uninitialized variable `SequenceNumber old_saved_seqno` causes asan related compilation error/warning below:

```
db_stress_tool/expected_state.cc:308:55: error: ‘old_saved_seqno’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  308 |   if (s.ok() && old_saved_seqno != kMaxSequenceNumber &&
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
```

Fix it by initializing to 0.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9265

Test Plan:
- make clean && COMPILE_WITH_ASAN=1 make -j48 db_stress_tool/expected_state.o
- monitor if same error happens again after merging

Reviewed By: ajkr

Differential Revision: D32939630

Pulled By: hx235

fbshipit-source-id: 41697515fd11ada8427f606b5dceb4e58d12cb80

db_stress_tool/expected_state.cc

@@ -273,7 +273,7 @@ Status FileExpectedStateManager::SaveAtAndAfter(DB* db) {
     s = FileSystem::Default()->RenameFile(state_file_temp_path, state_file_path,
                                           IOOptions(), nullptr /* dbg */);
   }
-  SequenceNumber old_saved_seqno;
+  SequenceNumber old_saved_seqno = 0;
   if (s.ok()) {
     old_saved_seqno = saved_seqno_;
     saved_seqno_ = seqno;