Fix nasty bug that would come up only if a client connection to a remote

Fix nasty bug that would come up only if a client connection to a remote
ldap server suddenly dies.
We were creating a wrong talloc hierarchy, so the event.fde was not
freed automatically as expected. This in turn made the event system call
the ldap io handlers with a null packet structure, causing a segfault.
Fix also the ordering in ldap_connection_dead()
Thanks to Metze for the huge help in tracking down this one.