ogg: Always alloc the private context in vorbis_header

It is possible to have an initial broken header and then valid packets.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
(cherry picked from commit 3562684db7)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/oggparsevorbis.c

@@ -212,15 +212,15 @@ vorbis_header (AVFormatContext * s, int idx)
     struct oggvorbis_private *priv;
     int pkt_type = os->buf[os->pstart];
 
-    if (!(pkt_type & 1))
-        return 0;
-
     if (!os->private) {
         os->private = av_mallocz(sizeof(struct oggvorbis_private));
         if (!os->private)
             return 0;
     }
 
+    if (!(pkt_type & 1))
+        return 0;
+
     if (os->psize < 1 || pkt_type > 5)
         return -1;