Avoid missing 'else' condition in getSubDirectories()
Avoid missing 'else' condition in getSubDirectories()

fs.readdir() either gives an error, or an array of values.
Other outcomes are not possible.

If we want to support that possibility, perhaps there should be
another `if` with `throw new Error('invalid fs.readdir value');`.
But, instead, I think maybe it should be removed.

yuanchuan/node-watch