Fix parsed is null error

[tomaka]

cc @expenses

This JS code hasn't really been properly reviewed by anyone unfortunately, and jumped a bit suddenly from "quickly written code to prototype" to "thing that we expect to be robust".

One consequence is that for some reason we checked parsed != null only after using parsed.

[mxinden]

For what my review on JS code is worth, this looks good to me.

thing that we expect to be robust

Why reinvent the wheel instead of using https://github.com/multiformats/js-multiaddr ? It would introduce some kind of dependency management though.

[tomaka]

Why reinvent the wheel instead of using https://github.com/multiformats/js-multiaddr ?

Note that we're supporting the x-parity-ws and x-parity-wss protocols (extensions so that one can add a URL to WebSocket addresses, while the exact format is being debated upstream), which aren't a thing in js-multiaddr.