bpo-38400 Don't check for NULL linked list pointers in _PyObject_IsFreed (GH-16630)

bpo-38400 Don't check for NULL linked list pointers in _PyObject_IsFreed (GH-16630)

Some objects like Py_None are not initialized with conventional means
that prepare the circular linked list pointers, leaving them unlinked
from the rest of the objects. For those objects, NULL pointers does
not mean that they are freed, so we need to skip the check in those
cases.
(cherry picked from commit 36e33c360ed7716a2b5ab2b53210da81f8ce1295)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>