plugins/elements/gstmultiqueue.c: Update queue state _after_ adding the item in the queue because else we could end u...

Original commit message from CVS:
* plugins/elements/gstmultiqueue.c: (gst_single_queue_flush),
(gst_multi_queue_chain), (gst_multi_queue_sink_event):
Update queue state _after_ adding the item in the queue because else we
could end up being full without the element added yet.