ogg: Do not try to use the parser if it is not present

Bug-Id: 886
CC: libav-stable@libav.org

libavformat/oggparsevorbis.c

@@ -364,6 +364,9 @@ static int vorbis_packet(AVFormatContext *s, int idx)
     struct oggvorbis_private *priv = os->private;
     int duration;
 
+    if (!priv->vp)
+        return AVERROR_INVALIDDATA;
+
     /* first packet handling
      * here we parse the duration of each packet in the first page and compare
      * the total duration to the page granule to find the encoder delay and