volume: return error if superblock is not initialized

fix https://github.com/chrislusf/seaweedfs/issues/1079

weed/storage/volume_loading.go

@@ -59,6 +59,9 @@ func (v *Volume) load(alsoLoadIndex bool, createDatIfMissing bool, needleMapKind
 	if alreadyHasSuperBlock {
 		e = v.readSuperBlock()
 	} else {
+		if !v.SuperBlock.Initialized() {
+			return fmt.Errorf("volume %s.dat not initialized", fileName)
+		}
 		e = v.maybeWriteSuperBlock()
 	}
 	if e == nil && alsoLoadIndex {

weed/storage/volume_super_block.go

@@ -69,6 +69,10 @@ func (s *SuperBlock) Bytes() []byte {
 	return header
 }
 
+func (s *SuperBlock) Initialized() bool {
+	return s.ReplicaPlacement == nil || s.Ttl == nil
+}
+
 func (v *Volume) maybeWriteSuperBlock() error {
 	stat, e := v.dataFile.Stat()
 	if e != nil {