talloc: ensure the sibling linked list remains valid during a free

talloc: ensure the sibling linked list remains valid during a free

This ensures that the sibling list of a pointer doesn't become invalid
during a free operation. It is an alternative fix to the fix in
6f51a1f45bf4de062cce7a562477e8140630a53d, and avoids the problem of
trying to calculate the parent pointer early

This should fix the subtle spoolss talloc bug that Simo found

Autobuild-User: Andrew Tridgell <tridge@samba.org>
Autobuild-Date: Tue Aug  9 01:53:17 CEST 2011 on sn-devel-104
(cherry picked from commit cf986f200804ce873b43c1ecf2d5e1bd08eb8a25)