ldb: validate ldb_dn_set_component input parameters even more strictly

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jelmer Vernooĳ <jelmer@samba.org>

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Jan  6 00:33:21 CET 2016 on sn-devel-144
