bug#9258: 24.0.50; image-next-line doesnt allow omitting arg

[Stefan Monnier]

> (defun image-next-line (&optional n)
>   "Scroll image in current window upward by N lines.
> Stop if the bottom edge of the image is reached."
>   (interactive "p")
>   (cond ((= n 0) nil)
>       ((< n 0)
>        (image-set-window-vscroll (max 0 (+ (window-vscroll) n))))
>       (t
>        (let* ((image (image-get-display-property))
>               (edges (window-inside-edges))
>               (win-height (- (nth 3 edges) (nth 1 edges)))
>               (img-height (ceiling (cdr (image-display-size image)))))
>          (image-set-window-vscroll (min (max 0 (- img-height win-height))
>                                         (+ n (window-vscroll))))))))

> declaration says n is optional but nil will fail on the numeric tests.
> so, either n isn't optional or the cond rearanged.

Please just make it mandatory.


        Stefan