AD: Use ad_site also when site search fails
Some deployments use the ad_site option for cases where the AD clients
are not able to find a site for one reason or another. With our current
code, the ad_site option value can only override a site that the client
found, not supply the value for cases no site could be found.
This patch fixes the issue.
Reviewed-by: Jakub Hrozek <firstname.lastname@example.org>