avformat/aviobuf: Honor avio_open[2] documentation

The documentation of both avio_open() as well as avio_open2() states
that on failure, the pointer to an AVIOContext given to this function
(via a pointer to a pointer to an AVIOContext) will be set to NULL. Yet
it didn't happen upon failure of ffurl_open_whitelist() or when allocating
the internal buffer failed. This commit changes this.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/aviobuf.c

@@ -1197,6 +1197,8 @@ int ffio_open_whitelist(AVIOContext **s, const char *filename, int flags,
     URLContext *h;
     int err;
 
+    *s = NULL;
+
     err = ffurl_open_whitelist(&h, filename, flags, int_cb, options, whitelist, blacklist, NULL);
     if (err < 0)
         return err;