Fix runtime crash as described on cython-devel.

__pyx_CyFunctionObject.defaults must be fully zeroed or a segfault or
memory corruption may occur if the object is visited by the garbage
collector before defaults is later populated. Uninitialized memory is
cast to PyObject* with undefined results:

    static int __Pyx_CyFunction_traverse(__pyx_CyFunctionObject *m, visitproc visit, void *arg)
    {
        ...
        if (m->defaults) {
            PyObject **pydefaults = __Pyx_CyFunction_Defaults(PyObject *, m);
            int i;
            for (i = 0; i < m->defaults_pyobjects; i++)
                Py_VISIT(pydefaults[i]);
                         ^^^^^^^^^^^^^