buffer overflow

[Chris Dollin]

My wrappers treat 0 as "" except compare(0,"")<0. It's arbitrary
in the same sense that O! = 1; a number of answers could be argued
correct, but one has to be chosen.

I'm not sure that 0! = X for X /= 1 could be argued correct, if one
wants to keep the identity N! = N x (N-1)! for N > 0.

`(char *) 0` isn't a string, it's the absence of a string (ignoring for
the moment that not all valid non-null char*'s are strings anyway).