sysinstall: refuse multiple requests

Otherwise, that triggers memory leak and double free.

src/sysinstall/sysinstall.c

@@ -1822,6 +1822,10 @@ static int vl_method_list_candidate_devices(
 
         assert(link);
 
+        /* Refuse multiple requests */
+        if (sd_varlink_get_userdata(link))
+                return -EBUSY;
+
         sd_varlink_server *varlink_server = sd_varlink_get_server(link);
         sd_event *event = sd_varlink_server_get_event(varlink_server);
         Hashmap **polkit_registry = ASSERT_PTR(sd_varlink_server_get_userdata(varlink_server));