be: remember last good server's name instead of fo_server structure
This fo_server may be freed when collapsing servers from SRV lookup
in `collapse_srv_lookup`. This would cause crash when we try to
dereference the pointer.
Resolves:
https://pagure.io/SSSD/sssd/issue/3976
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>