mpeg4: improve error checking while decoding packets.
mpeg4: improve error checking while decoding packets.

decode_picture() could return an error when an MPEG-4 profile is not
supported for example. In this case, the underlying VA context is not
allocated and no other proper action can be taken. Likewise on exit
from decode_slice().

Signed-off-by: Gwenole Beauchesne <729713629d9640cc095f1c77c692e1881f8093c4@intel.com>

GStreamer/gstreamer-vaapi,sreerenjb/sree-gstreamer-vaapi,CapOM/gstreamer-vaapi,sreerenjb/sree-gstreamer-vaapi,sreerenjb/sree-gstreamer-vaapi,ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi