plugins/elements/gstmultiqueue.c: Make sure we don't reference the buffer/event after we have given away ownership in...

Original commit message from CVS:
* plugins/elements/gstmultiqueue.c: (apply_buffer),
(gst_single_queue_push_one), (gst_multi_queue_chain),
(gst_multi_queue_sink_event):
Make sure we don't reference the buffer/event after we have given away
ownership in the queue.