avcodec/mjpegdec: Check that reference frame matches the current frame

Fixes: out of array read
Fixes: 2097/clusterfuzz-testcase-minimized-5036861833609216

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