gh-142495: make defaultdict keep existed value when racing with __missing__

[LindaSummer]

Issue

#142495

Proposed Changes

• Use PyDict_SetDefaultRef to guard the object update and keep existed value.
• Add a test case for the racing case

Comment

The PyDict_SetDefaultRef would increase the reference count of the setted item, so we must decrement the refrence count on __missing__ created object to avoid memory leak.

• Issue: defaultdict.__missing__ has data races #142495

[picnixz]

Suggested change

	PyObject* result;
	PyDict_SetDefaultRef(op, key, value, &result);
	// when PyDict_SetDefaultRef() < 0, result will be NULL
	// when PyDict_SetDefaultRef() == 0, result is a new reference of default value
	// when PyDict_SetDefaultRef() > 0, result is a new reference to the existing value
	// so the value reference must be decref'ed in all cases
	Py_DECREF(value);
	return result;
	PyObject *result = NULL;
	(void)PyDict_SetDefaultRef(op, key, value, &result);
	// 'result' is NULL, or a strong reference to 'value' or 'op[key]'
	Py_DECREF(value);
	return result;

[LindaSummer]

Hi @picnixz ,

Thanks very much for your suggestion! ❤
I have updated this part as suggested.

Best Regards,
Edward