Fix ThreadLocal on android/apple

Summary: The thread local destructor was assuming that the singleton object
had been created.

Reviewed By: @​yangchi

Differential Revision: D2536166

fb-gh-sync-id: b0c08e0990f684c0afae054ee17c62a924260f2b

folly/detail/ThreadLocalDetail.h

@@ -275,11 +275,12 @@ struct StaticMeta {
 #ifdef FOLLY_TLD_USE_FOLLY_TLS
     return &threadEntry_;
 #else
+    auto key = instance().pthreadKey_;
     ThreadEntry* threadEntry =
-        static_cast<ThreadEntry*>(pthread_getspecific(inst_->pthreadKey_));
+      static_cast<ThreadEntry*>(pthread_getspecific(key));
     if (!threadEntry) {
         threadEntry = new ThreadEntry();
-        int ret = pthread_setspecific(inst_->pthreadKey_, threadEntry);
+        int ret = pthread_setspecific(key, threadEntry);
         checkPosixError(ret, "pthread_setspecific failed");
     }
     return threadEntry;