avcodec/wcmv: check remaining space vs. blocks

Fixes: Timeout (18sec  -> 7sec)
Fixes: 14835/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WCMV_fuzzer-5646714897170432

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

libavcodec/wcmv.c

@@ -154,6 +154,9 @@ static int decode_frame(AVCodecContext *avctx,
         bytestream2_seek(&gb, 2, SEEK_SET);
     }
 
+    if (bytestream2_get_bytes_left(&gb) < 8LL * blocks)
+        return AVERROR_INVALIDDATA;
+
     if (s->prev_frame->data[0]) {
         ret = av_frame_copy(frame, s->prev_frame);
         if (ret < 0)