[bug 3064] Refix: Listbox fails to populate

[Ann & Jason Edmeades]

> LISTBOX_RemoveItem already does pretty much the same check, you should
> reuse that instead of duplicating it, you just need to remove the -1
> special case in there.

OK - Done, and changed the special case use by setcount appropriately

Changelog

Listbox delete string handling should validate the range of the index +
tests

Jason