My advice would be to not make a big thing of it, the more you think about it when you are in the conversation the more attention you are paying to it, and will probably end up stammering more. Don't think anything of it and carry on. When this happens to me personally, my first response (in my head) is along the lines of 'Ha, good joke', but I'll respond in person something like 'Really? That's reassuring, thanks', and by that point because I am not making such a conscious effort to hide my stammer I don't tend to trip up and stutter as much.