neosporin WITHOUT PAINKILLER is supposedly safe to use, just keeps the snake safe from infection. A little nip isn't anything to worry about unless it gets infected, which is something to watch out for because infections can be really bad, but easily preventable. Luckily I've never had to deal with it(knock on wood) since I've never had a snake that didn't take F/T or P/K(knock on wood again)
When feeding live, bites can happen, don't beat yourself up over it. If the bite was serious you'd be able to tell.