core: Fixes RTE while reorder LDAP servers list

Fixes RuntimeException appearing on LDAP servers list reordering. The
exception prevents user from login if the first LDAP server on list
wasn't been available.

Change-Id: Idc52415ca123790e967e0f1b940fdedb4987097a
Bug-Url: https://bugzilla.redhat.com/973566
Bug-Url: https://bugzilla.redhat.com/974148
Signed-off-by: Martin Perina <mperina@redhat.com>