avcodec/mpeg12dec: print the amount of overread

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg12dec.c

@@ -1949,8 +1949,10 @@ static int mpeg_decode_slice(MpegEncContext *s, int mb_y,
         }
     }
 eos: // end of slice
-    if (get_bits_left(&s->gb) < 0)
+    if (get_bits_left(&s->gb) < 0) {
+        av_log(s, AV_LOG_ERROR, "overread %d\n", -get_bits_left(&s->gb));
         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;