ga_getitem can fail if Py_GenericAlias returns NULL

[sobolevn]

Bug report

This code is problematic:

cpython/Objects/genericaliasobject.c

Lines 565 to 569 in dc03ce7

	PyObject *res = Py_GenericAlias(alias->origin, newargs);
	((gaobject *)res)->starred = alias->starred;
	Py_DECREF(newargs);
	return res;

Why? Py_GenericAlias can return NULL in different cases:

cpython/Objects/genericaliasobject.c

Lines 995 to 999 in dc03ce7

	return NULL;
	}
	if (!setup_ga(alias, origin, args)) {
	Py_DECREF(alias);
	return NULL;

So, we need to check for NULL.

Linked PRs

• gh-121660: Fix ga_getitem by explicitly checking for NULL result #121661
• [3.13] gh-121660: Fix ga_getitem by explicitly checking for NULL result (GH-121661) #121761
• [3.12] gh-121660: Fix ga_getitem by explicitly checking for NULL result (GH-121661) #121762