avcodec/mjpegdec: Check remaining bitstream in ljpeg_decode_yuv_scan()

Fixes timeout
Fixes: 445/fuzz-3-ffmpeg_VIDEO_AV_CODEC_ID_MJPEG_fuzzer
Fixes: 456/fuzz-2-ffmpeg_VIDEO_AV_CODEC_ID_JPEGLS_fuzzer

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