talloc: fixed a use after free error

talloc: fixed a use after free error

this is the minimal fix for the problem Rusty found. I previously
thought that the best fix would be to change tc->parent to be valid
for all pointers, but that is expensive for realloc with large numbers
of child pointers, which is much more commmon than I expected it to
be.

Autobuild-User: Andrew Tridgell <tridge@samba.org>
Autobuild-Date: Wed Jan  5 07:22:27 CET 2011 on sn-devel-104