8277441: CompileQueue::add fails with assert(_last->next() == __null) failed: not last

Reviewed-by: chagedorn, neliasso

src/hotspot/share/compiler/compileBroker.cpp

@@ -411,6 +411,7 @@ void CompileQueue::free_all() {
     CompileTask::free(current);
   }
   _first = NULL;
+  _last = NULL;
 
   // Wake up all threads that block on the queue.
   MethodCompileQueue_lock->notify_all();