Fix async load complete when state is destroyed
Fix async load complete when state is destroyed

Avoid raising this following error if state has been destroyed during async loading:
```
Uncaught TypeError: Cannot read property 'loadComplete' of null
    at Phaser.Loader.finishedLoading
    at Phaser.Loader.processLoadQueue
    at Phaser.Loader.asyncComplete
    at Phaser.Loader.fileComplete
    at Image.file.data.onload
```
photonstorm/phaser-ce,photonstorm/phaser-ce,yupaul/phaser-ce,yupaul/phaser-ce,photonstorm/phaser-ce,photonstorm/phaser-ce,yupaul/phaser-ce,yupaul/phaser-ce