avcodec/mjpegdec: Check for for the bitstream end in mjpeg_decode_scan_progressive_ac()

Fixes timeout
Fixes: 496/clusterfuzz-testcase-5805083497332736

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