prevent crash when destroying sprites in an anim callback

[Geokureli]

fixes #2782

The following example will crash without this fix

import flixel.FlxG;
import flixel.FlxSprite;

class AnimCallbackTestState extends flixel.FlxState
{
    override function create()
    {
        var sprite:FlxSprite = new FlxSprite().makeGraphic(100, 100);
        sprite.animation.add("anim", [0, 0, 0, 0], 4, false);
        add(sprite);
        
        // insert animation code here
        sprite.animation.finishCallback = function(anim:String)
        {
            sprite.destroy();
        }
        
        sprite.animation.play('anim');
    }
}

The issue is that parent is referenced internally after the finishCallback is fired. Specifically the callback fired in set_curFrame from FlxAnimation.hx#L218 and parent is referenced in FlxAnimation.hx#L220