Fully initialize allocated memory.

__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]);
                       ^^^^^^^^^^^^^

Cython/Utility/CythonFunction.c

@@ -554,7 +554,7 @@ static CYTHON_INLINE void *__Pyx_CyFunction_InitDefaults(PyObject *func, size_t
     m->defaults = PyMem_Malloc(size);
     if (!m->defaults)
         return PyErr_NoMemory();
-    memset(m->defaults, 0, sizeof(size));
+    memset(m->defaults, 0, size);
     m->defaults_pyobjects = pyobjects;
     return m->defaults;
 }