.. even with a self-compiled 1.2.3, INT_MAX/1000 ... nothing.
The problem is not crc32(), but the buffer itself:

   if (pbuf.len > 1024*5) {
        unsigned char *buf = pbuf.buf;
        Py_ssize_t len = pbuf.len;
        Py_ssize_t i;
fprintf(stderr, "CRC 32 2.1\n");
for(i=0; (size_t)i < (size_t)len;++i)
    *buf++ = 1;
fprintf(stderr, "CRC 32 2.2\n");

2.2 is never reached (in fact accessing buf[1] already causes fault).
Thus the problem is not zlib, but PyArg_ParseTuple().
But just don't ask me more on that!