Fix async load complete when state is destroyed

[clesquir]

This PR:

• is a bug fix

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

[samme]

This would handle the case where you call game.destroy() within a state preload callback?

[clesquir]

My actual case is when the user navigates to another page while the loader has not finished. The destroy would indeed be called before this.game.state.loadComplete();.

Thanks for merging this :)

[samme]

I wonder if we should just exit immediately in Loader#finishedLoading. There's no point in continuing if the game is destroyed. Or would users expect onLoadComplete to trigger in any case?