Bugfix 682 animation nullref

This is a bugfix for #682
Note: `FlxSprite::clone(sourceSprite)` was changed to `FlxSprite::clone(destinationSprite)` and now returns `destinationSprite` instead of `this`. `FlxAnimationController::clone()` was similarly modified to match 'clone to' (as `FlxVector::clone(destinationVector)`) convention.

Corresponding changes were added to `FlxTrail`.