FIX: make sure we call register_stops when changing schedules

This one is slightly tricky: when closing the schedule window, convoi_t::step calls set_schedule with fpl == f, but the schedule has still changed; a copy of the old schedule is stored in old_fpl, and this patch very temporarily restores it to compare against that instead.

Without the patch, you could cause a fatal index error by doing this:
 * create three stops: A, B, and C
 * create a line connecting A to B
 * create a convoy with a schedule containing A, B, and C
 * wait for the convoy to visit C
 * edit the convoy's schedule to consist only of A and B (it will automatically revert to being on the line)
 * wait for the convoy to visit the next stop

At that point, the departures map would contain an entry with iter.key.entry == 2, but the schedule would contain only two stops. This line in convoi_t::laden would then cause the error:

    const koord3d halt_position = fpl->eintrag.get_element(iter.key.entry).pos;

A better fix might be to make the schedule window operate on a copy of the schedule, not the actual convoy's schedule, but I've gone for the easiest fix.

I'm pretty sure this isn't present in Standard.