s3: libads: Don't re-do DNS lookups in ads_current_time() if not needed.

s3: libads: Don't re-do DNS lookups in ads_current_time() if not needed.

ADS_STRUCT may be being reused after a
DC lookup from ads_find_dc(), so ads->ldap.ss may already have a
good address (even if ads->server.ldap_server == NULL).
Only re-initialize the ADS_STRUCT and redo the ads_find_fc()
DNS lookups if we have to.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>