IDMAP: Prevent colision for explicitly defined slice.
Currently it happens for default domain, if someone configures
different ldap_idmap_default_domain_sid for two domains in
sssd.conf. There is no check preventing this in sdap_idmap.c, it's simply:
sdap_idmap_add_domain(idmap_ctx, dom_name,sid_str, 0). However, I believe
here is the best place to check it since there may be different use of
sss_idmap_calculate_ranges in the future.
Reviewed-by: Sumit Bose <sbose@redhat.com>