BUILD: connection: silence a couple of null-deref build warnings at -Wextra

These ones don't need to be checked either.
diff --git a/include/proto/connection.h b/include/proto/connection.h
index 85c5b63..31f82b8 100644
--- a/include/proto/connection.h
+++ b/include/proto/connection.h
@@ -1029,7 +1029,7 @@
  */
 static inline int conn_install_mux_fe(struct connection *conn, void *ctx)
 {
-	struct bind_conf     *bind_conf = objt_listener(conn->target)->bind_conf;
+	struct bind_conf     *bind_conf = __objt_listener(conn->target)->bind_conf;
 	const struct mux_ops *mux_ops;
 
 	if (bind_conf->mux_proto)