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: Chen Hanxiao <chenhanxiao@cn.fujitsu.com>

daemon/remote.c

@@ -4643,7 +4643,8 @@ lxcDispatchDomainOpenNamespace(virNetServerPtr server ATTRIBUTE_UNUSED,
 cleanup:
     if (rv < 0)
         virNetMessageSaveError(rerr);
-    virDomainFree(dom);
+    if (dom)
+        virDomainFree(dom);
     return rv;
 }