procqueue.c: rely on item type instead of its position

In the osmo_gapk_pq_prepare() we do allocate an item's buffer
conditionally, only when its type is not sink, because an output
buffer is not required for sink.

Let's use a bit more elegant way to check, whether item is sink.

Change-Id: I770a1d02273d9d8301a9e4ec72426fb8f4060277
1 file changed