fix eventTarget emit bug

Related issue: https://github.com/GoodBoyDigital/pixi.js/issues/1131

http://jsfiddle.net/rz553zr3/

Before:
````js
var listeners = this._listeners[eventName],
    length = listeners.length,
    fn = listeners[0],
    i;

for(i = 0; i < length; fn = listeners[++i]) {
    //call the event listener
    fn.call(this, data);
````
If the callback function included a off() call of the event, the listeners variable will be updated and the for loop call will be messed up.