Fix bug: Trying to convert undefined to string

Fix bug when trying to convert malformed message.y (undefined) to string.
    
    /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)