ThreadLocalDetail: fix bug just introduced w/ recent change to constexpr-ctor style class

Summary: Small, silly bug which is triggered when there are no keys to unregister, invalidating the current size of the structure.

Reviewed By: yzhan

Differential Revision: D2712604

fb-gh-sync-id: f6f031936d8e4e458b49c08714f42cb2eec4c4fe

folly/detail/ThreadLocalDetail.h

@@ -184,8 +184,8 @@ class PthreadKeyUnregister {
 
   ~PthreadKeyUnregister() {
     std::lock_guard<std::mutex> lg(mutex_);
-    while (size_--) {
-      pthread_key_delete(keys_[size_]);
+    while (size_) {
+      pthread_key_delete(keys_[--size_]);
     }
   }