MDL-46671 availability: DEBUG when course_module does not exist.

There was a fatal error when calendar event linked to non-extant activities.

Now, instead of throwing an exception when a course_module does not exist, is_user_visible() will print a debug statement.

availability/classes/info_module.php

@@ -180,7 +180,12 @@ public static function is_user_visible($cmorid, $userid = 0, $checkcourse = true
             if (is_object($cmorid)) {
                 $cmorid = $cmorid->id;
             }
-            $cm = $DB->get_record('course_modules', array('id' => $cmorid), '*', MUST_EXIST);
+            $cm = $DB->get_record('course_modules', array('id' => $cmorid));
+            if (!$cm) {
+                // In some error cases, the course module may not exist.
+                debugging('info_module::is_user_visible called with invalid cmid ' . $cmorid, DEBUG_DEVELOPER);
+                return false;
+            }
         }
 
         // If requested, check user can access the course.