bpo-38070: visit_decref() calls _PyObject_IsFreed() (GH-15782)

In debug mode, visit_decref() now calls _PyObject_IsFreed() to ensure
that the object is not freed. If it's freed, the program fails with
an assertion error and Python dumps informations about the freed
object.

Modules/gcmodule.c

@@ -376,6 +376,8 @@ static int
 visit_decref(PyObject *op, void *data)
 {
     assert(op != NULL);
+    _PyObject_ASSERT(op, !_PyObject_IsFreed(op));
+
     if (PyObject_IS_GC(op)) {
         PyGC_Head *gc = AS_GC(op);
         /* We're only interested in gc_refs for objects in the