Fix error code check in releaseBuffer

BufferQueueConsumer can occasionaly return STALE_BUFFER_SLOT which
does not indicate an error but that there is no more to be done.
SurfaceFlingerConsumer should report and error only with a status
code inferior to zero.

Test: AUPT
Change-Id: Ie0624358301e1e3b090adf35facd6eac79c214db