If the drain function of the decoder triggered by FLUSH_START is run while the output loop is running, once the output loop finished vtdec->downstream_ret will be GST_FLOW_FLUSHING instead of GST_FLOW_OK, which must not be treated as an error since the queue is cleaned correctly as well. Fix #4179 Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8328>