Use the right variable when unserializing old attendee data

plugins/calendar/drivers/database/database_driver.php

@@ -1450,7 +1450,7 @@ class database_driver extends calendar_driver
     }
     // decode the old serialization format
     else {
-      foreach (explode("\n", $event['attendees']) as $line) {
+      foreach (explode("\n", $s_attendees) as $line) {
         $att = array();
         foreach (rcube_utils::explode_quoted_string(';', $line) as $prop) {
           list($key, $value) = explode("=", $prop);