multiqueue: never consider a queue that is not waiting

multiqueue: never consider a queue that is not waiting

.. when computing the high id.

After a flush for instance, sq->srcresult is reset to OK,
yet it doesn't make sense to pick a non-existing position
id as the high id when a queue doesn't contain any items
in that situation either.

It is in any case completely OK to let the not-linked stream
get consumed without throttling at this stage, as any
first packet arriving on other single queues will get assigned
a higher position id.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/885>