Fix obscure breakage (relative to 2.3) in listsort: the test for list

mutation during list.sort() used to rely on that listobject.c always
NULL'ed ob_item when ob_size fell to 0.  That's no longer true, so the
test for list mutation during a sort is no longer reliable.  Changed the
test to rely instead on that listobject.c now never NULLs-out ob_item
after (if ever) ob_item gets a non-NULL value.  This new assumption is
also documented now, as a required invariant in listobject.h.

The new assumption allowed some real simplification to some of the
hairier code in listsort(), so is a Good Thing on that count.

Include/listobject.h

@@ -30,6 +30,9 @@ typedef struct {
      *     0 <= ob_size <= allocated
      *     len(list) == ob_size
      *     ob_item == NULL implies ob_size == allocated == 0
+     *     If ob_item ever becomes non-NULL, it remains non-NULL for the
+     *         life of the list object.  The check for mutation in list.sort()
+     *         relies on this odd detail.
      */
     int allocated;
 } PyListObject;