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>
432ada66f2