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

gh-121660: Fix `ga_getitem` by explicitly checking for `NULL` result (GH-121661)
(cherry picked from commit bb802db8cf)

Co-authored-by: sobolevn <mail@sobolevn.me>

Objects/genericaliasobject.c

@@ -564,6 +564,10 @@ ga_getitem(PyObject *self, PyObject *item)
     }
 
     PyObject *res = Py_GenericAlias(alias->origin, newargs);
+    if (res == NULL) {
+        Py_DECREF(newargs);
+        return NULL;
+    }
     ((gaobject *)res)->starred = alias->starred;
 
     Py_DECREF(newargs);