avoid nil DataBackend

weed/storage/volume_vacuum.go

@@ -91,9 +91,11 @@ func (v *Volume) CommitCompact() error {
 
 	glog.V(3).Infof("Got volume %d committing lock...", v.Id)
 	v.nm.Close()
+	if v.DataBackend != nil {
 		if err := v.DataBackend.Close(); err != nil {
 			glog.V(0).Infof("fail to close volume %d", v.Id)
 		}
+	}
 	v.DataBackend = nil
 	stats.VolumeServerVolumeCounter.WithLabelValues(v.Collection, "volume").Dec()