appsink: fix end condition of query drain handler

appsink: fix end condition of query drain handler

The while loop should end when all buffers "and" the preroll
buffer are consumed but this means to continue waiting if there
are still some pending buffers "or" preroll buffer.

The unit test was correct but racy because of this mistake.
I.e. because of the wrong "and" the while could finish too early.

cd tests/check && GST_CHECKS=test_query_drain make elements/appsink.forever

https://bugzilla.gnome.org/show_bug.cgi?id=789763