发现个bug，Tween缺clear()函数，导致出错

[SteveCruise]

Joystick的destroy内容如下：

 _destroy() {
    if (this.tweener) {
      this.tweener.clear();
      this.tweener = null;
    }
  }

它调用了Tween实例的clear()但是Tween类并没有clear()，导致渲染循环中的Update()执行出错

VM702:1 gameThirdScriptError
Cannot read property 'position' of null
TypeError: Cannot read property 'position' of null
    at Sprite.set (http://127.0.0.1:19127/game/libs/pixi.js:5724:48)
    at Function.update (http://127.0.0.1:19127/game/src/base/Tween.js:94:37)
    at App._update (http://127.0.0.1:19127/game/src/App.js:335:23)
    at App.loop (http://127.0.0.1:19127/game/src/App.js:341:12)