Fix crash on resize while doing other things

Commits 9ffe33a2 and
4352df00 introduce and use a
getScreenLineColumns(line) method to provide support for DECDWL
(Double-Width) lines.

It turns out that under some conditions on resize Screen::_cuY (the
current cursor Y position) and ScreenWindow::endWindowLine() can have
different ideas of how many lines the terminal has.

A test that asserts:

 - while [ true ]; do echo -e "\e[?1047h";  done
 - Resize the window, making it smaller

BUG: 436327