…rator

Problem:  borrow_stl_vsep_hl() takes the status line highlight from the
    cell before the vertical separator, which a window of zero width does
    not have: the cell before the screen is read.  With the windows laid
    out for a size the GUI has not reached, the rows below the screen are
    read as well, which crashes gvim on MS-Windows in test_popup.vim
    since the tests redraw the shell after setting its size.
Solution: Skip a window of zero width and stay inside the screen.
    Test_statusline_vsep_borrow_zero_width_window() puts a zero by zero
    window next to another one with both status lines on the first
    screen line, which reads before the screen without the fix.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>