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

libavcodec/mjpegdec.c

@@ -1082,6 +1082,10 @@ static int ljpeg_decode_yuv_scan(MJpegDecodeContext *s, int predictor,
 
     for (mb_y = 0; mb_y < s->mb_height; mb_y++) {
         for (mb_x = 0; mb_x < s->mb_width; mb_x++) {
+            if (get_bits_left(&s->gb) < 1) {
+                av_log(s->avctx, AV_LOG_ERROR, "bitstream end in yuv_scan\n");
+                return AVERROR_INVALIDDATA;
+            }
             if (s->restart_interval && !s->restart_count){
                 s->restart_count = s->restart_interval;
                 resync_mb_x = mb_x;