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.

Objects/object.c

@@ -454,9 +454,12 @@ _PyObject_IsFreed(PyObject *op)
     /* ignore op->ob_ref: its value can have be modified
        by Py_INCREF() and Py_DECREF(). */
 #ifdef Py_TRACE_REFS
-    if (_PyMem_IsPtrFreed(op->_ob_next) || _PyMem_IsPtrFreed(op->_ob_prev)) {
+    if (op->_ob_next != NULL && _PyMem_IsPtrFreed(op->_ob_next)) {
         return 1;
     }
+    if (op->_ob_prev != NULL && _PyMem_IsPtrFreed(op->_ob_prev)) {
+         return 1;
+     }
 #endif
     return 0;
 }