bug#4040: 23.1; before-string overlay causes error when scrolling

[Chong Yidong]

> Evaluating this form signals an error:
>
> ELISP> (with-temp-buffer
>        (switch-to-buffer (current-buffer))
>        (insert-char ?x (1+ (window-width)))
>        (let ((ovl (make-overlay (1- (point)) (point))))
>          (overlay-put ovl 'before-string " "))
>        (goto-char 1)
>        (scroll-up 1))
> *** Eval error ***  Args out of range: 0
>
> It comes from get_char_property_and_overlay, which is called with
> position == 0 by pos_visible_p.

Thanks, I've checked in a fix.