In case lastNode==0, need to set it to -1
In case lastNode==0, need to set it to -1

Fix https://github.com/chrislusf/seaweedfs/issues/156

chrislusf/seaweedfs,chrislusf/seaweedfs,hxiaodon/seaweedfs,hxiaodon/seaweedfs,k85169336/seaweedfs,chrislusf/seaweedfs,k85169336/seaweedfs,chrislusf/seaweedfs