ldb: make the 'spy' code more paranoid

ldb: make the 'spy' code more paranoid

the spy code in ldb_tdb was added a while ago to overcome a memory
hierarchy problem with async ldb errors. Recently we started to get
valgrind errors related to the order of free in the spy code. This
patch ensures that we don't try to use a freed spy pointer. This
prevents the valgrind errors, although I suspect that the memory
hierarchy we have here is more complex than it needs to be

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>

Autobuild-User: Andrew Tridgell <tridge@samba.org>
Autobuild-Date: Thu Sep  1 08:54:23 CEST 2011 on sn-devel-104