queue2: fix event/preroll deadlock

queue2: fix event/preroll deadlock

The qlock is released between popping a buffer from the queue
and pushing it. When this buffer causes the sink to wait in
preroll, this lets a query see that the queue is empty, and
push the query then wait for it to be serviced. However, this
will not be done till after peroll, and this will thus block.
If upstream was waiting on buffering to reach 100% before
switching to PLAYING, a deadlock would ensue.

We fix it by refusing the query when buffering, as per Wim's
recommendation on IRC.