MDL-31599 block_news - incorrect RSS feed for guest user

Was previously using the userid 0, rather than guest to use
authenticated RSS feed.

Thanks to Paul Verrall at LUNS for helping me debug this.

blocks/news_items/block_news_items.php

@@ -106,7 +106,7 @@ class block_news_items extends block_base {
                     $tooltiptext = get_string('rsssubscriberssposts','forum');
                 }
                 if (!isloggedin()) {
-                    $userid = 0;
+                    $userid = $CFG->siteguest;
                 } else {
                     $userid = $USER->id;
                 }