avformat/omadec: simplify checks in nprobe()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mjpegenc: dont store DQT for LJPEG

This may improve compatibility of lgpegs generated by libavcodec
also encoded ljpegs become slightly smaller

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