Only add defined callbacks to the stack

[darrachequesne]

The kind of change this PR does introduce

• a bug fix

Current behaviour

Before that commit, undefined callbacks were also added to the array,
which could lead to 'Maximum call stack size exceeded' error when
flush() method was called.

New behaviour

Other information (e.g. related issues)

Fixes #399
Fixes socketio/socket.io#2589

[3rd-Eden]

Looks to me that you're fixing symptoms here instead of the actual bug, you probably want to prevent undefined's from ending up in that array in the first place.

[darrachequesne]

@3rd-Eden my reasoning is that when using socket.send('a'), callback arg is undefined, right?

Socket.prototype.send =
Socket.prototype.write = function (data, options, callback) {
  this.sendPacket('message', data, options, callback);
  return this;
};

[darrachequesne]

@3rd-Eden or have I misunderstood your comment?

[darrachequesne]

Steps to reproduce:

var opts = { allowUpgrades: false, transports: ['websocket'] };
var engine = listen(opts, function (port) {
  var socket = new eioc.Socket('ws://localhost:%d'.s(port), { transports: ['websocket'] });
  engine.on('connection', function (conn) {
    for (var i = 0; i < 200000; i++) {
      conn.send('a');
    }
  });
});