avoid possible nil when node is disconnected from its parent

fix https://github.com/chrislusf/seaweedfs/issues/2073

weed/topology/data_node.go

@@ -177,7 +177,13 @@ func (dn *DataNode) GetVolumesById(id needle.VolumeId) (vInfo storage.VolumeInfo
 
 func (dn *DataNode) GetDataCenter() *DataCenter {
 	rack := dn.Parent()
+	if rack == nil {
+		return nil
+	}
 	dcNode := rack.Parent()
+	if dcNode == nil {
+		return nil
+	}
 	dcValue := dcNode.GetValue()
 	return dcValue.(*DataCenter)
 }