fix error after stopping tween inside onUpdate

It is possible to stop tween inside onUpdate callback. So it changes internal _tweens array and it's length, and cached length in TWEEN.update becomes obsolete. And if this tween wasn't last, there will be error.

In this example:
```javascript
var tween  = new TWEEN.Tween({})
var second = new TWEEN.Tween({})

tween.onUpdate(function() { tween.stop() })

tween .to({}, 500).start()
second.to({}, 500).start()

TWEEN.update()
```

a "TypeError: Cannot call method 'update' of undefined" will be thrown