daemon: don't free domain if it's null

If we fail to get domain, we had to judge whether
it's null or not when doing 'cleanup'.
Signed-off-by: NChen Hanxiao <chenhanxiao@cn.fujitsu.com>

daemon/remote.c

@@ -4643,6 +4643,7 @@ lxcDispatchDomainOpenNamespace(virNetServerPtr server ATTRIBUTE_UNUSED,
 cleanup:
     if (rv < 0)
         virNetMessageSaveError(rerr);
+    if (dom)
         virDomainFree(dom);
     return rv;
 }