Merge pull request #2527 from orivej/fix-py_import-null-va_arg

Fix calling va_arg function with a NULL argument from generated C++ code

Cython/Utility/ImportExport.c

@@ -65,7 +65,7 @@ static PyObject *__Pyx_Import(PyObject *name, PyObject *from_list, int level) {
             if (!py_level)
                 goto bad;
             module = PyObject_CallFunctionObjArgs(py_import,
-                name, global_dict, empty_dict, list, py_level, NULL);
+                name, global_dict, empty_dict, list, py_level, (PyObject *)NULL);
             Py_DECREF(py_level);
             #else
             module = PyImport_ImportModuleLevelObject(