This looks like a va_* related bug to me.
None of the va_* arguments would seem to be used before we hit the fault.
The arg list to ApacheCookie_new() must be NULL-terminated, and between the r and NULL there must be an even number of arguments.
The call to ApacheCookie_new() in XS_Apache__Cookie_parse() is OK:
c = ApacheCookie_new(r, NULL);
I'm using gcc 3.3.2. Will leave a build of gcc 3.4.3 running overnight and try rebuilding apache with that sometime tomorrow.
Carl