.d raises crashes when trying to add in node labels

```{R}
if (is.null(x$phy$node.label)) 
        x$phy$node.label <- apply(expand.grid(letters, letters, 
            letters), 1, paste, collapse = "")[seq_len(tree$Nnode)]
```

...should be...

```{R}
if (is.null(x$phy$node.label)) 
        x$phy$node.label <- apply(expand.grid(letters, letters, 
            letters), 1, paste, collapse = "")[seq_len(x$phy$Nnode)]
```