Long lines at the end of the buffer cause an exception

[lzybkr]

Easy repro on Windows - set a small height/width, e.g. 40x40. Hit enter until the prompt is on the last line, then enter a long line that forces the buffer to scroll (without pasting). You'll get an exception.

The bug was introduced by #790 - so this only repros post-beta3.

When fixing, be sure to verify editing, e.g. deleting text so the command line no longer spans multiple lines is also fixed.