Include items on exterior stairs of custom houses in MultiRef.items

[KevinEady]

Items on exterior stairs of custom houses were not being included in the multi's item script member.

Here is a script where I target the robe, check robe.multi, and loop through robe.multi.items:

    var item := Target(who).multi;
    Broadcast($"multi: {item}");
    foreach child_item in (item.items)
        Broadcast($"  child item: {child_item.objtype} - {child_item.desc}");
    endforeach

First iteration shows the robe not included, and second iteration shows robe included:

[coveralls]

Coverage: 44.589% (-0.009%) from 44.599% when pulling ccf053c on increase-multi-items-bounds-check into d9fd8d4 on master.

[turleypol]

foo? :)
A comment why these two versions exists would be good when we cannot get rid of the second version, otherwise we will forget it.