avcodec/mjpegdec: Check for bits left in mjpeg_decode_scan_progressive_ac()

Fixes: Timeout
Fixes: 36262/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_JPEGLS_fuzzer-4969052454912000

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


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 909faca9)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>