[3.14] gh-142975: During GC, mark frozen objects with a merged zero refcount for destruction (GH-143156) (GH-143175)

gh-142975: During GC, mark frozen objects with a merged zero refcount for destruction (GH-143156)
(cherry picked from commit 8611f74e08)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

Python/gc_free_threading.c

@@ -905,7 +905,11 @@ exit:
 static void
 queue_untracked_obj_decref(PyObject *op, struct collection_state *state)
 {
-    if (!_PyObject_GC_IS_TRACKED(op)) {
+    assert(Py_REFCNT(op) == 0);
+    // gh-142975: We have to treat frozen objects as untracked in this function
+    // or else they might be picked up in a future collection, which breaks the
+    // assumption that all incoming objects have a non-zero reference count.
+    if (!_PyObject_GC_IS_TRACKED(op) || gc_is_frozen(op)) {
         // GC objects with zero refcount are handled subsequently by the
         // GC as if they were cyclic trash, but we have to handle dead
         // non-GC objects here. Add one to the refcount so that we can