avcodec/mpeg12dec: Check for overread in mpeg_decode_slice()

This is needed in case the checked bitstream reader is disabled
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg12dec.c

@@ -1846,6 +1846,8 @@ static int mpeg_decode_slice(MpegEncContext *s, int mb_y,
         }
     }
 eos: // end of slice
+    if (get_bits_left(&s->gb) < 0)
+        return AVERROR_INVALIDDATA;
     *buf += (get_bits_count(&s->gb)-1)/8;
     av_dlog(s, "y %d %d %d %d\n", s->resync_mb_x, s->resync_mb_y, s->mb_x, s->mb_y);
     return 0;