defaulting file ttl to volume ttl

fix https://github.com/chrislusf/seaweedfs/issues/992

weed/storage/needle/volume_ttl.go

@@ -48,6 +48,9 @@ func ReadTTL(ttlString string) (*TTL, error) {
 
 // read stored bytes to a ttl
 func LoadTTLFromBytes(input []byte) (t *TTL) {
+	if input[0] == 0 && input[1] == 0 {
+		return EMPTY_TTL
+	}
 	return &TTL{Count: input[0], Unit: input[1]}
 }