MDL-35238 Accept $CFG->alternativeupdateproviderurl from config.php file only

There was a potential security risk that someone with access to the Moodle
database could update mdl_config table and use it as a vector to
install malicious code on the server.

Credit goes to Dan Poltawski for raising this.

lib/pluginlib.php

@@ -987,8 +987,8 @@ class available_update_checker {
     protected function prepare_request_url() {
         global $CFG;
 
-        if (!empty($CFG->alternativeupdateproviderurl)) {
-            return $CFG->alternativeupdateproviderurl;
+        if (!empty($CFG->config_php_settings['alternativeupdateproviderurl'])) {
+            return $CFG->config_php_settings['alternativeupdateproviderurl'];
         } else {
             return 'http://download.moodle.org/api/1.1/updates.php';
         }