gh-128889: Zero out memory ctypes for generated struct layout tests (GH-128944)

Author: encukou

Lib/test/test_ctypes/test_generated_structs.py

@@ -443,7 +443,7 @@ def test_generated_data(self):
         - None
         - reason to skip the test (str)
 
-        This does depend on the C compiler keeping padding bits zero.
+        This does depend on the C compiler keeping padding bits unchanged.
         Common compilers seem to do so.
         """
         for name, cls in TESTCASES.items():
@@ -696,7 +696,8 @@ def output(string):
             output('                ' + line)
         typename = f'{struct_or_union(cls)} {name}'
         output(f"""
-                {typename} value = {{0}};
+                {typename} value;
+                memset(&value, 0, sizeof(value));
                 APPEND(PyUnicode_FromString({c_str_repr(name)}));
                 APPEND(PyLong_FromLong(sizeof({typename})));
                 APPEND(PyLong_FromLong(_Alignof({typename})));