Uncaught error in index.js / utils.walkdir

If the user doesn't have permission on a file/dir, `utils.walkdir` results in an `Uncaught TypeError` because `res` is null.

```
if (stats && stats.isDirectory()) {
  utils.walkdir(filepath, base, function(err, res) {
    res.forEach(function(dirEntry) {
      results.push(dirEntry);
    });
    next();
  });
}
```