stop when dn.parent is nil

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

weed/topology/topology_event_handling.go

@@ -56,5 +56,7 @@ func (t *Topology) UnRegisterDataNode(dn *DataNode) {
 	dn.UpAdjustVolumeCountDelta(-dn.GetVolumeCount())
 	dn.UpAdjustActiveVolumeCountDelta(-dn.GetActiveVolumeCount())
 	dn.UpAdjustMaxVolumeCountDelta(-dn.GetMaxVolumeCount())
+	if dn.Parent() != nil {
 		dn.Parent().UnlinkChildNode(dn.Id())
 	}
+}