Fix build with newer compilers on FreeBSD

[rhurlin]

On FreeBSD current the installation process of databases/py-postgresql is broken [1]. optimized.cpython-39.so is not built most likely because due to compiler errors:

    In file included from postgresql/port/_optimized/module.c:43:
    postgresql/port/_optimized/buffer.c:590:2: error: incompatible pointer to integer conversion initializing 'Py_ssize_t' (aka 'long') with an expression of type 'void *' [-Wint-conversion]
            NULL,                                                                   /* tp_print */
            ^~~~
    /usr/include/sys/_null.h:34:14: note: expanded from macro 'NULL'
                    ^~~~~~~~~~~

The same happens with postgresql/port/_optimized/wirestate.c:251:2 and is reported on FreeBSD's Bugzilla [2].

The patch tries to solve the problem by setting the value of NULL to 0 for the argument tp_print. This seems to work as expected, hopefully without regressions.

[1] https://portsfallout.com/fallout?port=databases%2Fpy-postgresql&maintainer=&env=main-amd64-default&category=&flavor=
[2] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271165

[jwp]

That is indeed the fix. Thank you.