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

[joakim]

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Please write in English if possible, as the Emacs maintainers
usually do not have translators for other languages.

Please describe exactly what actions triggered the bug, and
the precise symptoms of the bug.  If you can, give a recipe
starting from `emacs -Q':


(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.

                                          
If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
For information about debugging Emacs, please read the file
/usr/local/share/emacs/24.0.50/etc/DEBUG.


In GNU Emacs 24.0.50.20 (x86_64-unknown-linux-gnu, GTK+ Version 3.0.11)
 of 2011-08-03 on localhost.localdomain
Windowing system distributor `Fedora Project', version 11.0.11003000
configured using `configure  '--with-xwidgets' '--without-gnutls' 
'--enable-asserts' '--with-x-toolkit=gtk3''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=none
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Emacs-Lisp

Minor modes in effect:
  shell-dirtrack-mode: t
  predictive-which-dict-mode: t
  paredit-mode: t
  desktop-save-mode: t
  erc-truncate-mode: t
  erc-track-mode: t
  erc-track-minor-mode: t
  erc-spelling-mode: t
  erc-ring-mode: t
  erc-pcomplete-mode: t
  erc-netsplit-mode: t
  erc-log-mode: t
  erc-button-mode: t
  erc-fill-mode: t
  erc-autojoin-mode: t
  erc-autoaway-mode: t
  ido-everywhere: t
  erc-stamp-mode: t
  erc-match-mode: t
  show-paren-mode: t
  erc-smiley-mode: t
  erc-irccontrols-mode: t
  eldoc-mode: t
  recentf-mode: t
  grab-and-drag-mode: t
  yas/global-mode: t
  yas/minor-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t
  abbrev-mode: t

Recent input:

-- 
Joakim Verona