nn_mul_kara causes out-of-bounds reads about bsdnt HOT 4 CLOSED

I'll take a look and see where it's going wrong. Note that I have to work on this in my spare time, of which I have little these days. So unless it's easy to give a correct fix, this might take me a while. I'm of course happy to merge patches though, and see you've submitted a PR.

I got curious about how such a severe error could have ended up in the code and tracked it down. The nn_mul_kara function was actually doing some work it didn't need to, so I removed that. It's not clear whether I should have left it in, since an extra ASSERT could be added to check the limb that was being computed was in fact zero. But for performance, and to make the code cleaner, I removed it. The extra limb would never be used anywhere, it was just being computed, presumably for the sake of an ASSERT. Let me know what you prefer here. After that, it was strictly using the wrong length for the final addition. I've now fixed this quite severe error in master. I'm actually really surprised this didn't show up in the tests anywhere. It's really rather bad.