assertion to prevent crash

pandatool/src/pstatserver/pStatReader.cxx

@@ -308,6 +308,7 @@ void PStatReader::
 dequeue_frame_data() {
   while (!_queued_frame_data.empty()) {
     const FrameData &data = _queued_frame_data.front();
+    nassertv(_client_data != (PStatClientData *)NULL); 
 
     // Check to see if any new collectors have level data.
     int num_levels = data._frame_data->get_num_levels();