avcodec/mjpegdec: Check for end of bitstream in ljpeg_decode_rgb_scan()

Fixes: Timeout
Fixes: 8648/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MJPEG_fuzzer-5108395525799936

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

libavcodec/mjpegdec.c

@@ -1055,6 +1055,11 @@ static int ljpeg_decode_rgb_scan(MJpegDecodeContext *s, int nb_components, int p
         for (mb_x = 0; mb_x < s->mb_width; mb_x++) {
             int modified_predictor = predictor;
 
+            if (get_bits_left(&s->gb) < 1) {
+                av_log(s->avctx, AV_LOG_ERROR, "bitstream end in rgb_scan\n");
+                return AVERROR_INVALIDDATA;
+            }
+
             if (s->restart_interval && !s->restart_count){
                 s->restart_count = s->restart_interval;
                 resync_mb_x = mb_x;