multiqueue: do not reset last push result when pushing events

Use the last result as a default when pushing a item from a single queue,
otherwise the status gets reset to _OK when pushing events.

This causes problems when mistakenly activating a not-linked stream
that is being ignored upstream as it is not being used (adaptive
scenarios), it will make the multiqueue post a buffering message
on a pad that won't receive buffers

https://bugzilla.gnome.org/show_bug.cgi?id=725917

plugins/elements/gstmultiqueue.c

@@ -1067,7 +1067,7 @@ static GstFlowReturn
 gst_single_queue_push_one (GstMultiQueue * mq, GstSingleQueue * sq,
     GstMiniObject * object)
 {
-  GstFlowReturn result = GST_FLOW_OK;
+  GstFlowReturn result = sq->srcresult;
 
   if (GST_IS_BUFFER (object)) {
     GstBuffer *buffer;