Fix build on FreBSD

Summary:
kross-interpreters/python/cxx/Objects.hxx:1081:15: error: cannot initialize
      a member subobject of type 'int' with an rvalue of type 'nullptr_t'
            , offset( NULL )
              ^       ~~~~

Reviewers: #freebsd, aspotashev

Differential Revision: https://phabricator.kde.org/D6928