resolved: introduce dns_server_is_fallback

src/resolve/resolved-dns-server.c

@@ -100,9 +100,7 @@ int dns_server_new(
         /* A new DNS server that isn't fallback is added and the one
          * we used so far was a fallback one? Then let's try to pick
          * the new one */
-        if (type != DNS_SERVER_FALLBACK &&
-            m->current_dns_server &&
-            m->current_dns_server->type == DNS_SERVER_FALLBACK)
+        if (type != DNS_SERVER_FALLBACK && dns_server_is_fallback(m->current_dns_server))
                 manager_set_dns_server(m, NULL);
 
         if (ret)

src/resolve/resolved-dns-server.h

@@ -182,4 +182,8 @@ void dns_server_unref_stream(DnsServer *s);
 
 DnsScope *dns_server_scope(DnsServer *s);
 
+static inline bool dns_server_is_fallback(DnsServer *s) {
+        return s && s->type == DNS_SERVER_FALLBACK;
+}
+
 int dns_server_dump_state_to_json(DnsServer *server, sd_json_variant **ret);