Fix bug #6098 - When the DNS server is invalid, the ads_find_dc() does not work corre...

Fix bug #6098 - When the DNS server is invalid, the ads_find_dc() does not work correctly with "security = domain"

1. If DNS server is invalid, the get_sorted_dc_list() is called with
realm(FQDN) and it fails.
2. On the next step, the get_sorted_dc_list() is called with realm(FQDN) again.

I think "again" is wrong place.
On the 2nd step, get_sorted_dc_list() should be called with realm(WORKGROUP).
(cherry picked from commit 2f3eafae83d036c5b35e5d7d11be6e29aaab7533)

Signed-off-by: Michael Adam <obnox@samba.org>