stop when dn.parent is nil
stop when dn.parent is nil

fix https://github.com/chrislusf/seaweedfs/issues/473

hxiaodon/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs,k85169336/seaweedfs,chrislusf/seaweedfs,hxiaodon/seaweedfs,chrislusf/seaweedfs,k85169336/seaweedfs