avcodec/wmv2dec: skip frames that have only skiped MBs

This requires us to pre-parse the skip data, as we want to
detect this before allocating all the arrays

Fixes: Timeout
Fixes: 9708/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMV2_fuzzer-5729709861109760

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

@@ -141,6 +141,21 @@ int ff_wmv2_decode_picture_header(MpegEncContext *s)
     if (s->qscale <= 0)
         return AVERROR_INVALIDDATA;
 
+    if (s->pict_type != AV_PICTURE_TYPE_I && show_bits(&s->gb, 1)) {
+        GetBitContext gb = s->gb;
+        int skip_type = get_bits(&gb, 2);
+        int run = skip_type == SKIP_TYPE_COL ? s->mb_width : s->mb_height;
+
+        while (run > 0) {
+            int block = FFMIN(run, 25);
+            if (get_bits(&gb, block) + 1 != 1<<block)
+                break;
+            run -= block;
+        }
+        if (!run)
+            return FRAME_SKIPPED;
+    }
+
     return 0;
 }