varlink: properly allocate connection event source

Let's make sure we keep a reference to the event source

(Note that this code is currently not used, which is why this was never
used: in all cases we do not add listener fds after the event is
attached, but before. In that case this code is not called.)

src/shared/varlink.c

@@ -2212,9 +2212,7 @@ int varlink_server_listen_fd(VarlinkServer *s, int fd) {
         };
 
         if (s->event) {
-                _cleanup_(sd_event_source_unrefp) sd_event_source *es = NULL;
-
-                r = sd_event_add_io(s->event, &es, fd, EPOLLIN, connect_callback, ss);
+                r = sd_event_add_io(s->event, &ss->event_source, fd, EPOLLIN, connect_callback, ss);
                 if (r < 0)
                         return r;