avformat/matroskadec: ignore CodecPrivate if the stream is VP9

Defined in a recent revision of https://www.webmproject.org/docs/container/

This prevents storing the contents of CodecPrivate into extradata for
a codec that doesn't need nor expect any. It will among other things
prevent matroska specific binary data from being dumped onto other
formats during remuxing.
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskadec.c

@@ -2397,6 +2397,10 @@ static int matroska_parse_tracks(AVFormatContext *s)
                 return ret;
         } else if (codec_id == AV_CODEC_ID_PRORES && track->codec_priv.size == 4) {
             fourcc = AV_RL32(track->codec_priv.data);
+        } else if (codec_id == AV_CODEC_ID_VP9 && track->codec_priv.size) {
+            /* we don't need any value stored in CodecPrivate.
+               make sure that it's not exported as extradata. */
+            track->codec_priv.size = 0;
         }
         track->codec_priv.size -= extradata_offset;