Fix bug when message.y is undefined

/Users/alexander/trrnts/node_modules/bittorrent-dht/client.js:533
  var type = message.y.toString()
                       ^
TypeError: Cannot call method 'toString' of undefined
    at DHT._onData (/Users/alexander/trrnts/node_modules/bittorrent-dht/client.js:533:24)
    at Socket.EventEmitter.emit (events.js:98:17)
    at UDP.onMessage (dgram.js:440:8)