Ogg: discard non-essential metadata from Vorbis header when creating extradata

The first part of the metadata, the "vendor" string, is required by
libvorbis, it will refuse to play when it is not available.
Also we do not currently parse that part into metadata so it would also
be lost if we removed it as well.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/oggparsevorbis.c

@@ -256,8 +256,16 @@ vorbis_header (AVFormatContext * s, int idx)
             st->time_base.den = srate;
         }
     } else if (os->buf[os->pstart] == 3) {
-        if (os->psize > 8)
-            ff_vorbis_comment (s, &st->metadata, os->buf + os->pstart + 7, os->psize - 8);
+        if (os->psize > 8 &&
+            ff_vorbis_comment(s, &st->metadata, os->buf + os->pstart + 7, os->psize - 8) >= 0) {
+            // drop all metadata we parsed and which is not required by libvorbis
+            unsigned new_len = 7 + 4 + AV_RL32(priv->packet[1] + 7) + 4 + 1;
+            if (new_len >= 16 && new_len < os->psize) {
+                AV_WL32(priv->packet[1] + new_len - 5, 0);
+                priv->packet[1][new_len - 1] = 1;
+                priv->len[1] = new_len;
+            }
+        }
     } else {
         st->codec->extradata_size =
             fixup_vorbis_headers(s, priv, &st->codec->extradata);