Fix req.subdomains returns when passing IP address as hostname

Currently, when passing an ip address like `127.0.0.1` as hostname, `req.subdomains` returns an array with address parts:  `['0', '127']`. I tried to fix it by returning an empty array instead.

Not sure, however, whether you prefer adding an external dependency (like validator - https://github.com/chriso/validator.js) to check for IP address. (The used regex are taken from it by the way.)