MDL-31607 use full course object for require_login() if already available

This eliminates 1 db query.

mod/chat/gui_basic/index.php

@@ -41,7 +41,7 @@ if (!$cm = get_coursemodule_from_instance('chat', $chat->id, $course->id)) {
 }
 
 $context = get_context_instance(CONTEXT_MODULE, $cm->id);
-require_login($course->id, false, $cm);
+require_login($course, false, $cm);
 require_capability('mod/chat:chat', $context);
 $PAGE->set_pagelayout('base');