Correct logic for stopping people using guest access from doing quizzes. Merged from MOODLE_18_STABLE.

mod/quiz/attempt.php

@@ -108,7 +108,7 @@
     }
 
 /// Check availability
-    if (isguest()) {
+    if (isguestuser()) {
         print_heading(get_string('guestsno', 'quiz'));
         if (empty($popup)) {
             print_footer($course);

mod/quiz/view.php

@@ -137,7 +137,7 @@
     //
     // So for courses that allow guest access, it is good to offer people an easy
     // way to log in at this point if they have got this far before logging in.
-    if (isguest()) {
+    if (isguestuser()) {
         $loginurl = $CFG->wwwroot.'/login/index.php';
         if (!empty($CFG->loginhttps)) {
             $loginurl = str_replace('http:','https:', $loginurl);