[C1] Error in calendar when deleting user that has registries in agenda_event_invitee

[NicoDucou]

In Chamilo 1.11.x when deleting a user the registry agenda_event_invitee.user_id take a NULL value, but this generate error "Uncaught Error: Call to a member function getId() on null" in main/inc/lib/agenda.lib.php:2135

So to avoid this problem and to have a better coherence in the content in the database if the user is deleted then the registries in agenda_event_invitee related to this user should be deleted to.