avcodec/mjpegdec: Check for unsupported bayer case

Fixes: out of array access
Fixes: 51462/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TIFF_fuzzer-662559341582745

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>