BufferQueueProducer: Slot may have null buffer

I'm not entirely sure how we got in to this state
but it seems best not to crash. It seems like we
are tearing down and everything will work out
in the end anyway.

Bug: 153913134
Test: Existing tests pass
Change-Id: I1d21fa5f29ca1478ecf8e7cd6363f6f176ae9bee