Don't reload users for dummy sessions

The service for a dummy session will be NULL. If authentication fails for
a dummy session, then no service level actions should be taken.

Only the binlogrouter can trigger authentication failure with a dummy
session as it creates connections before the service itself has started.

server/core/service.c

@@ -1606,6 +1606,7 @@ service_update(SERVICE *service, char *router, char *user, char *auth)
  */
 int service_refresh_users(SERVICE *service)
 {
+    ss_dassert(service);
     int ret = 1;
 
     if (spinlock_acquire_nowait(&service->spin))