[3.12] gh-105375: Harden error handling in _testcapi/heaptype.c (GH-105608) (#105615)

Bail on first error in heapctypesubclasswithfinalizer_finalize()
(cherry picked from commit d636d7d)

Co-authored-by: Erlend E. Aasland <[email protected]>

Author: miss-islington

Modules/_testcapi/heaptype.c

@@ -661,8 +661,11 @@ heapctypesubclasswithfinalizer_finalize(PyObject *self)
         goto cleanup_finalize;
     }
     oldtype = PyObject_GetAttrString(m, "HeapCTypeSubclassWithFinalizer");
+    if (oldtype == NULL) {
+        goto cleanup_finalize;
+    }
     newtype = PyObject_GetAttrString(m, "HeapCTypeSubclass");
-    if (oldtype == NULL || newtype == NULL) {
+    if (newtype == NULL) {
         goto cleanup_finalize;
     }