avcodec/ituh263dec: Avoid spending a long time in slice sync

Fixes: 177/fuzz-3-ffmpeg_VIDEO_AV_CODEC_ID_FLV1_fuzzer

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

libavcodec/ituh263dec.c

@@ -167,6 +167,7 @@ static int h263_decode_gob_header(MpegEncContext *s)
         /* We have a GBSC probably with GSTUFF */
     skip_bits(&s->gb, 16); /* Drop the zeros */
     left= get_bits_left(&s->gb);
+    left = FFMIN(left, 32);
     //MN: we must check the bits left or we might end in an infinite loop (or segfault)
     for(;left>13; left--){
         if(get_bits1(&s->gb)) break; /* Seek the '1' bit */