Fix buffer overrun bug in scrolling code

When attempting to scroll from the cursor, with the cursor outside of the
scrolling window, the program would crash.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Hardcode more tricky cases for fuzz test

Insert mode and scroll windows easily lead to bugs.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>