Initialize uninitialized std::atomic variables

Summary: Initialize uninitialized std::atomic variables

Reviewed By: yfeldblum

Differential Revision: D9758050

fbshipit-source-id: 865d89eddafc81f3cab6f11e2ebb669f7ff70d04

db/db_test_util.h

@@ -575,7 +575,7 @@ class SpecialEnv : public EnvWrapper {
 
   std::atomic<bool> is_wal_sync_thread_safe_{true};
 
-  std::atomic<size_t> compaction_readahead_size_;
+  std::atomic<size_t> compaction_readahead_size_{};
 };
 
 class MockTimeEnv : public EnvWrapper {