#66 crash on -sl 0 on super small windows

[Chris Alfeld]

If -sl 0 is passed and more text is displayed than can
fit on screen aterm crashes.

Thus
aterm -sl 0 -geometry 10x10 -e echo Hello
works fine.

But
aterm -sl 0 -geometry 2x1 -e echo Hello
gives

aterm: screen.c:872: scr_add_lines: Assertion
`screen.cur.row >= -TermWin.nscrolled' failed.
Abort (core dumped)

Workaround: There is no problem when '-sl 1' is used,
even with large amounts of text.