MDL-59909 calendar: Try hard not to fail the adhoc task.

Triggering a fatal error in an adhoc task is bad. It will be retried indefinitely.
Even though we are not sure how to get a module instance without a course module record,
it is possible and should not kill the Moodle site.

completion/classes/api.php

@@ -59,7 +59,10 @@ class api {
         if (is_object($instanceorid)) {
             $instance = $instanceorid;
         } else {
-            $instance = $DB->get_record($modulename, array('id' => $instanceorid), '*', MUST_EXIST);
+            $instance = $DB->get_record($modulename, array('id' => $instanceorid), '*', IGNORE_MISSING);
+        }
+        if (!$instance) {
+            return false;
         }
         $course = get_course($instance->course);