src: fix size underflow in CallbackQueue

Only decrease the size when actually removing items.

PR-URL: #34662
Fixes: #34657
Refs: #34572
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/callback_queue-inl.h

@@ -22,8 +22,8 @@ CallbackQueue<R, Args...>::Shift() {
     head_ = ret->get_next();
     if (!head_)
       tail_ = nullptr;  // The queue is now empty.
+    size_--;
   }
-  size_--;
   return ret;
 }