Fix server.close, fixes #900

lib/server.js

@@ -21,13 +21,13 @@ function Server (torrent, opts) {
   var _close = server.close
   server.close = function (cb) {
     closed = true
-    torrent = null
     server.removeListener('connection', onConnection)
     server.removeListener('request', onRequest)
     while (pendingReady.length) {
       var onReady = pendingReady.pop()
       torrent.removeListener('ready', onReady)
     }
+    torrent = null
     _close.call(server, cb)
   }