encodePeer in client.js doesn't check if port is our of range

[bjackson]

encodePeer doesn't check for out of bounds values before writing them to a Buffer, which can cause an error to be thrown.

I would submit a PR, but I'm not sure what the behavior should be when a peer with a port that is outside the range is discovered.

https://github.com/webtorrent/bittorrent-dht/blob/master/client.js#L618