My practice is to credit where I got the game assets in my game description (and in-game, if I have in-game credits), which I think is a nice thing to do anyway, but in this type of situation this informs the customers where you got the assets from and and that you're using them legitimately (and if another game also licensed them, now they know). There may still be people who cry foul without bothering to read the game description, but what can you do.