use Py_XSETREF when filling cache

Author: deadlovelll

Objects/descrobject.c

@@ -624,7 +624,10 @@ descr_get_qualname(PyObject *self, void *Py_UNUSED(ignored))
     PyObject *qualname;
     Py_BEGIN_CRITICAL_SECTION(self);
     if (descr->d_qualname == NULL) {
-        descr->d_qualname = calculate_qualname(descr);
+        PyObject *new_qualname = calculate_qualname(descr);
+        if (new_qualname != NULL) {
+            Py_XSETREF(descr->d_qualname, new_qualname);
+        }
     }
     qualname = Py_XNewRef(descr->d_qualname);
     Py_END_CRITICAL_SECTION();