It is not only unknown if this holds on machines that don't use 2s complement, it is quite likely that this doesn't even hold on machines that do use 2s complem

It is not only unknown if this holds on machines that don't use 2s complement, it is quite likely that this doesn't even hold on machines that do use 2s complement, but have an optimizing compiler. In many cases, the overflow checks contains terms like (b->slen+1) < 0 which an optimizing compiler could compile as b->slen < -1 effectively switching the overflow test off (as there is no non-overflowing execution where this code will produce other behaviour than the original code, and behaviour on overflow is undefined, so the compiler can safely get rid of the addition).