resolve: get the current DNS server before passing the server lists

manager_get_dns_server() may re-read /etc/resolv.conf and replace
m->dns_servers and m->search_domains, but it was passed as an argument
to dns_configuration_json_append() together with those lists, and the
order of evaluation of the arguments is unspecified. gcc with LTO reads
the list heads first, so after a foreign /etc/resolv.conf has changed
the freed lists are walked. Typically the serialization then fails with
EUCLEAN, which makes the SubscribeDNSConfiguration method of
io.systemd.Resolve.Monitor and in turn systemd-networkd-wait-online
--dns fail.

Call manager_get_dns_server() first and store the result in a local
variable.

(cherry picked from commit 04567a3b5e2e29a2252e89be8a3c625c692103d3)

src/resolve/resolved-manager.c

@@ -2173,12 +2173,16 @@ static int global_dns_configuration_json_append(Manager *m, sd_json_variant **co
         if (r < 0)
                 return r;
 
+        /* This may re-read /etc/resolv.conf and replace m->dns_servers and m->search_domains, hence call it
+         * before they are passed below, as the order of evaluation of the arguments is unspecified. */
+        DnsServer *current_dns_server = manager_get_dns_server(m);
+
         return dns_configuration_json_append(
                         /* ifname= */ NULL,
                         /* ifindex= */ 0,
                         /* delegate= */ NULL,
                         /* default_route= */ -1,
-                        manager_get_dns_server(m),
+                        current_dns_server,
                         m->dns_servers,
                         m->fallback_dns_servers,
                         m->search_domains,