Fix a bug in which we call Reflection::ClearField() on a default message instance.

PiperOrigin-RevId: 539740799

python/google/protobuf/pyext/repeated_scalar_container.cc

@@ -57,6 +57,7 @@ namespace repeated_scalar_container {
 
 static int InternalAssignRepeatedField(RepeatedScalarContainer* self,
                                        PyObject* list) {
+  cmessage::AssureWritable(self->parent);
   Message* message = self->parent->message;
   message->GetReflection()->ClearField(message, self->parent_field_descriptor);
   for (Py_ssize_t i = 0; i < PyList_GET_SIZE(list); ++i) {