SYSDB: Read the ldb_message from loop's index counter when reading subdomain UPNs
There was a typo in code that read the UPN suffixes from the subdomain
ldb_message. As a result, the UPN suffixes from the first domain were
always consulted for all domains.
Related to:
https://pagure.io/SSSD/sssd/issue/3431
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
Reviewed-by: Sumit Bose <sbose@redhat.com>
(cherry picked from commit a8a3fcbf6f75a7c2665e8bf503c186e07dfab333)