fix crash on invalid udp tracker response

Author: feross

lib/udp-tracker.js

@@ -82,6 +82,7 @@ UDPTracker.prototype._request = function (opts) {
   var socket = dgram.createSocket('udp4')
 
   var cleanup = function () {
+    if (!socket) return
     self._cleanupFns.splice(self._cleanupFns.indexOf(cleanup), 1)
     if (timeout) {
       clearTimeout(timeout)