bug#13066: 24.2.90; setting rmail-summary-window-size causes error

[Kenichi Handa]

When I customize rmail-summary-window-size to some integer
value (e.g. 10), M-x rmail-summary signals this error:

        Wrong type argument: stringp, nil

This is with emacs-24 branch.  This doesn't happen with
Emacs 23, but I don't remember it happens with 24.1 and 24.2.

The precise recipe:
% emacs -Q
M-x rmail RET
M-x customize-variable RET rmail-summary-window-size
;; Then set it to 10, click [apply], type q, then type h

In GNU Emacs 24.2.90.1 (i686-pc-linux-gnu, GTK+ Version 3.4.2)
 of 2012-11-28 on mongkok
Bzr revision: 108135 handa@gnu.org-20121128115844-3bprvblqksuw4xbg
Windowing system distributor `The X.Org Foundation', version 11.0.11103000
System Description:     Linux Mint 13 Maya

Important settings:
  value of $LC_MONETARY: ja_JP.UTF-8
  value of $LC_NUMERIC: ja_JP.UTF-8
  value of $LC_TIME: ja_JP.UTF-8
  value of $LANG: ja_JP.utf8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
  display-time-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t

Recent input:
M-x r m a i l <return> M-x c u s t o <tab> i <tab> 
- v a r <tab> <return> r m a i l - s u <tab> b u <tab> 
<backspace> <backspace> w i <tab> <return> <help-echo> 
<help-echo> <down-mouse-1> <help-echo> <down-mouse-1> 
<mouse-movement> <mouse-1> <backspace> <return> C-p 
C-e 1 o <backspace> 0 <help-echo> <help-echo> <help-echo> 
<down-mouse-1> <help-echo> <mouse-1> <help-echo> q 
h C-x o C-x b * M e <tab> <return> C-a C-SPC C-e M-w 
M-x r e p o r t <tab> <return>

Recent messages:
Resetting customization items...done
Creating customization setup...done
To install your edits, invoke [State] and choose the Set operation
Change the state of this item.
Computing summary lines...690
rmail-summary: Wrong type argument: stringp, nil
Mark set
Scanning for dabbrevs...done
user-error: No dynamic expansion for `rmail-summary-wi' found
Quit

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr dabbrev emacsbug qp
rmailkwd help-fns cus-edit cus-start cus-load wid-edit
rmailmm message format-spec rfc822 mml mml-sec mm-decode
mm-bodies mm-encode mailabbrev gmm-utils mailheader
mail-parse rfc2231 time sendmail rmail-spam-filter easymenu
rmailsum rmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr
mail-utils time-date japan-util tooltip ediff-hook vc-hooks
lisp-float-type mwheel x-win x-dnd tool-bar dnd fontset
image regexp-opt fringe tabulated-list newcomment lisp-mode
register page menu-bar rfn-eshadow timer select scroll-bar
mouse jit-lock font-lock syntax facemenu font-core frame
cham georgian utf-8-lang misc-lang vietnamese tibetan thai
tai-viet lao korean japanese hebrew greek romanian slovak
czech european ethiopic indian cyrillic chinese case-table
epa-hook jka-cmpr-hook help simple abbrev minibuffer
loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget hashtable-print-readable
backquote make-network-process dynamic-setting
system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty emacs)