fix build failure (#484)

diff --git a/protocol/block.go b/protocol/block.go
index 5260575..68e3776 100755 (executable)
--- a/protocol/block.go
+++ b/protocol/block.go
@@ -151,7 +151,8 @@ func (c *Chain) SaveBlock(block *types.Block) error {
                return err
        }
 
-       log.WithFields(log.Fields{"height": block.Height, "hash": block.Hash().String()}).Info("Block saved on disk")
+       blockHash := block.Hash()
+       log.WithFields(log.Fields{"height": block.Height, "hash": blockHash.String()}).Info("Block saved on disk")
        return nil
 }