avoid nil exception

fix https://github.com/chrislusf/seaweedfs/issues/1869

weed/storage/needle_map_leveldb.go

@@ -152,9 +152,11 @@ func (m *LevelDbNeedleMap) Close() {
 		glog.Warningf("close index file %s failed: %v", indexFileName, err)
 	}
 
+	if m.db != nil {
 		if err := m.db.Close(); err != nil {
 			glog.Warningf("close levelDB failed: %v", err)
 		}
+	}
 }
 
 func (m *LevelDbNeedleMap) Destroy() error {