nvenc: Early terminate handle_frame if the last flow was not GST_FLOW_OK
If the last flow was not GST_FLOW_OK, the encoding thread is not running and there is nothing to pop from GAsyncQueue (this causes deadlock).
To prevent deadlock, just return the handle_frame without further encoding process if the last flow was not GST_FLOW_OK. Note that the last flow will be cleared per FLUSH_STOP and STREAM_START event.