Of course, you can guard against that particular case. But the point is that SRP has pitfalls, just like every other solution. And those pitfalls aren't well known; the Wikipedia pseudocode makes no mention of that exploit, for example.
1. Carol will abort if she receives B == 0 (mod N) or u == 0.
2. Steve will abort if he receives A (mod N) == 0.
3. Carol must show her proof of K first. If Steve detects that Carol's proof is incorrect, he must abort without showing his own proof of K.
[1] Ok, the python code doesn't seem to, you're correct. However, that's less a demonstration of a protocol implementation, and more a demonstration of the protocol's math. The page does mention it though in the protocol section. It would be appropriate (and maybe later I'll do this) to break that out so it's more obvious.
Alice: At the time I thought Eve was the only one I had to worry about. Little
did I know, Carol would be the one who'd really replace me in the end.