avcodec/wmv2dec: Check input bits vs. coded MBs

Fixes: Timeout (94sec ->8sec)
Fixes: 14387/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMV2_fuzzer-5723546887651328

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

libavcodec/wmv2dec.c

@@ -33,6 +33,7 @@
 static int parse_mb_skip(Wmv2Context *w)
 {
     int mb_x, mb_y;
+    int coded_mb_count = 0;
     MpegEncContext *const s = &w->s;
     uint32_t *const mb_type = s->current_picture_ptr->mb_type;
 
@@ -83,6 +84,14 @@ static int parse_mb_skip(Wmv2Context *w)
         }
         break;
     }
+
+    for (mb_y = 0; mb_y < s->mb_height; mb_y++)
+        for (mb_x = 0; mb_x < s->mb_width; mb_x++)
+            coded_mb_count += !IS_SKIP(mb_type[mb_y * s->mb_stride + mb_x]);
+
+    if (coded_mb_count > get_bits_left(&s->gb))
+        return AVERROR_INVALIDDATA;
+
     return 0;
 }