Fixes issue in patch dnsmasq-2.79-server-domain-rh1919894.patch.
When /etc/resolv.conf is changed, dnsmasq reloads used servers. But it
does not call cleanup of server domains in this case. It might cause
serv_domain->last_server to become non-null, but pointing released
server. Ensure it is checked before any cleanup_servers() action always
and from all other places, like dbus setting.
Caused unending loop in forward_query function, rhbz#2106361.
Resolves: rhbz#2120711
