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/cache.c

@@ -310,7 +310,7 @@ static int cache_close(URLContext *h)
             av_log(h, AV_LOG_ERROR, "Could not delete %s.\n", c->filename);
         av_freep(&c->filename);
     }
-    ffurl_close(c->inner);
+    ffurl_closep(&c->inner);
     av_tree_enumerate(c->root, NULL, NULL, enu_free);
     av_tree_destroy(c->root);