avoid nil
avoid nil

fix https://github.com/chrislusf/seaweedfs/issues/452

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