Commit 7f2df64649 converted `mission_menu` to inherit from `listbox`,
but introduced an off-by-one bug in the handling of subdirectories.
`listbox` must be told how many strings it is given. Subdirectories
have one extra string, to represent the `listbox_go_up` element. The
count passed to `listbox` incorrectly failed to adjust for the generated
go-up element, causing listbox not to show the last string in the list.
Change the logic to adjust the count to include the extra element.
Fixes: 7f2df64649 ("Make mission_menu inherit from listbox")
Reported-by: AlumiuN <https://github.com/dxx-rebirth/dxx-rebirth/issues/629>
87cc4c79c8