multiqueue: do not reset last push result when pushing events

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