Bugs fixed

Fixes #8574 - while iterating over the `illness` vector, the "lying down" disease adds a new entry "sleep" into that vector which invalidates the iterators. By using a index to iterate, this is avoided. Changing the vector to list or set is also a possibility, but I was under impression diseases should be migrated to  Creature-effects - is that right?

Fixes #8571 - not the PR, but the underlying problem. `calendar::season_length` uses  `ACTIVE_WORLD_OPTIONS` which is emptied after unloading the current world (see `worldfactory::set_active_world`), when accessing it, the default constructed option has the value 0, which triggered a division-by-zero.