Check context before accessing its field.

Patch by Thorsten Jordan: tjordan macrosystem de

Originally committed as revision 10939 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avio.c

@@ -133,6 +133,7 @@ offset_t url_seek(URLContext *h, offset_t pos, int whence)
 int url_close(URLContext *h)
 {
     int ret;
+    if (!h) return 0; /* can happen when url_open fails */
 
     ret = h->prot->url_close(h);
     av_free(h);