Properly init collectDone_

Summary: facepaw

Reviewed By: djwatson

Differential Revision: D2709030

fb-gh-sync-id: 20fe50bc70e97c4fedeae8fd271a4de70fa0e805

folly/experimental/TLRefCount.h

@@ -159,7 +159,7 @@ class TLRefCount {
 
     std::mutex collectMutex_;
     Int collectCount_{0};
-    bool collectDone_;
+    bool collectDone_{false};
   };
 
   std::atomic<State> state_{State::LOCAL};