smacker: remove unnecessary call to avctx->release_buffer in decode_frame()
The release_buffer was cleaning the provided frame, thus causing the
successive call to avctx->reget_buffer() to allocate a new frame. In
case the returned frame was not the same one previously returned but a
new one with different data, it resulted in artifacts.
Signed-off-by: Ronald S. Bultje <firstname.lastname@example.org>