Initialize std::atomic<std::thread::id> SingletonHolder::creating_thread_

Summary: Initialize `std::atomic<std::thread::id> SingletonHolder::creating_thread_`.

Reviewed By: yfeldblum

Differential Revision: D9525334

fbshipit-source-id: 93831c40bed21d1f3356bd2181e78ba08b7b116f

folly/Singleton.h

@@ -344,7 +344,7 @@ struct SingletonHolder : public SingletonHolderBase {
   std::atomic<SingletonHolderState> state_{SingletonHolderState::NotRegistered};
 
   // the thread creating the singleton (only valid while creating an object)
-  std::atomic<std::thread::id> creating_thread_;
+  std::atomic<std::thread::id> creating_thread_{};
 
   // The singleton itself and related functions.