Fix event listener leak check timing

[koichik]

Because check for listener leak is performed before the listener was added to an array, the warning occurred the 12th listener not the 11th listener.

Reproduce:

$ cat leak.js
var events = require('events');
var emitter = new events.EventEmitter();
for (var i = 0; i < 10; ++i) {
  emitter.on('data', function() {});
}
console.log('added 10 listeners');
console.log('adding 11th listener');
emitter.on('data', function() {});
console.log('adding 12th listener');
emitter.on('data', function() {});

$ node leak.js
added 10 listeners
adding 11th listener
adding 12th listener
(node) warning: possible EventEmitter memory leak detected. 11 listeners added. Use emitter.setMaxListeners() to increase limit.
Trace: 
    at EventEmitter.<anonymous> (events.js:123:17)
    at Object.<anonymous> (/tmp/leak.js:10:9)
    at Module._compile (module.js:404:26)
    at Object..js (module.js:410:10)
    at Module.load (module.js:336:31)
    at Function._load (module.js:297:12)
    at Array.0 (module.js:423:10)
    at EventEmitter._tickCallback (node.js:126:26)