avformat: Replace ffurl_close() by ffurl_closep() where appropriate

It avoids leaving dangling pointers behind in memory.

Also remove redundant checks for whether the URLContext to be closed is
already NULL.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/rtmpcrypt.c

@@ -240,7 +240,7 @@ static int rtmpe_close(URLContext *h)
     RTMPEContext *rt = h->priv_data;
 
     ff_dh_free(rt->dh);
-    ffurl_close(rt->stream);
+    ffurl_closep(&rt->stream);
 
     return 0;
 }