ffmpeg_filter: fix pointer to local outside scope

Fixes CID1206652
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/truemotion2: replace impossible condition by av_assert2

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mjpegdec: Fix undefined shift

Fixes CID1194388

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>