8033696: "assert(thread != NULL) failed: just checking" due to Thread::current() and JNI pthread interaction

Reviewed-by: dholmes, dsamersoff

hotspot/src/share/vm/runtime/vmThread.cpp

@@ -305,6 +305,9 @@ void VMThread::run() {
     _terminate_lock->notify();
   }
 
+  // Thread destructor usually does this.
+  ThreadLocalStorage::set_thread(NULL);
+
   // Deletion must be done synchronously by the JNI DestroyJavaVM thread
   // so that the VMThread deletion completes before the main thread frees
   // up the CodeHeap.