MDL-52975 mnet: Set system context on the MNet SSO landing page

Without the context being explicitly set, debugging warning can be
displayed in certain rare situations.

auth/mnet/land.php

@@ -35,6 +35,7 @@
 $url = new moodle_url('/auth/mnet/jump.php', array('token'=>$token, 'idp'=>$remotewwwroot, 'wantsurl'=>$wantsurl));
 if ($wantsremoteurl !== false) $url->param('remoteurl', $wantsremoteurl);
 $PAGE->set_url($url);
+$PAGE->set_context(context_system::instance());
 
 $site = get_site();