fix panic 5435 (#5436)

weed/topology/volume_layout.go

@@ -301,7 +301,7 @@ func (vl *VolumeLayout) PickForWrite(count uint64, option *VolumeGrowOption) (vi
 			if float64(info.Size) > float64(vl.volumeSizeLimit)*option.Threshold() {
 				shouldGrow = true
 			}
-			return vid, count, locationList, shouldGrow, nil
+			return vid, count, locationList.Copy(), shouldGrow, nil
 		}
 		return 0, 0, nil, shouldGrow, errors.New("Strangely vid " + vid.String() + " is on no machine!")
 	}

weed/topology/volume_location_list.go

@@ -28,6 +28,9 @@ func (dnll *VolumeLocationList) Copy() *VolumeLocationList {
 
 func (dnll *VolumeLocationList) Head() *DataNode {
 	//mark first node as master volume
+	if dnll.Length() == 0 {
+		return nil
+	}
 	return dnll.list[0]
 }