samlogon_cache: Simplify netsamlogon_cache_have

We're interested in existence only, we should be able to trust the data
format consistency for this type of query.

netsamlogon_cache_get calls netsamlogon_cache_init for us, now we have
to do it directly.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
