avcodec/mjpegdec: Check for rgb before flipping

Fixes assertion failure due to unsupported case

Fixes: 356/fuzz-1-ffmpeg_VIDEO_AV_CODEC_ID_MJPEG_fuzzer
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
