don't update end time to past values, fixes #111

controllers/sheets.php

@@ -1512,7 +1512,7 @@ class SheetsController extends StudipController
 
             // extend exam duration for already active participants
             if ($assignment_type === 'exam' && $assignment->options['duration'] != $exam_length) {
-                $sql = 'UPDATE vips_assignment_attempt SET end = end + INTERVAL ? MINUTE
+                $sql = 'UPDATE vips_assignment_attempt SET end = GREATEST(end + INTERVAL ? MINUTE, NOW())
                         WHERE assignment_id = ? AND end > NOW()';
                 $stmt = $db->prepare($sql);
                 $stmt->execute([$exam_length - $assignment->options['duration'], $assignment_id]);