BufferQueueProducer: fix which slot gets pre-allocated

Commit 78014f32 introduced a bug that
made us pre-allocate buffers into the last available free slots instead
of the first available ones. This in turn caused more re-allocations,
and possibly triggered driver bugs.

Change-Id: Ic4a70e676b4f2bbb054bc873be62ced26e3099a0