libavformat/oggparsevorbis: Fix memleak on multiple headers

Fixes: Chromium bug 800123
Reported-by: Matt Wolenetz <wolenetz@google.com>
Reviewed-by: Matt Wolenetz <wolenetz@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 3934aa495d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/oggparsevorbis.c

@@ -228,6 +228,10 @@ static int fixup_vorbis_headers(AVFormatContext *as,
 
     len = priv->len[0] + priv->len[1] + priv->len[2];
     buf_len = len + len / 255 + 64;
+
+    if (*buf)
+        return AVERROR_INVALIDDATA;
+
     ptr = *buf = av_realloc(NULL, buf_len);
     if (!ptr)
         return AVERROR(ENOMEM);