mmal/codec: Remove useless locking

The mmal_queue is threadsafe and the block released from output callback
does not interfere with the block processing in decode. What could
happen was a use after free of block when checking the decode loop
condition after submitting the block to mmal. Avoid this by clearing the
block pointer before submitting to mmal and use it as extra loop
condition.

Signed-off-by: Julian Scheel <julian@jusst.de>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>