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.