bug#5000: 23.1.50; empty string not allowed as line-prefix

[Carsten Dominik <carsten.dominik@gmail.com>]

It seems that an empty string as a line prefix makes the redisplay
engine choke.

Test case:

- open a new buffer in fundamental mode, no auto-fill-mode
- insert a long line that will be wrapped for display into two or more
   lines.
- insert the following form into the buffer, after the long line

    (add-text-properties (point-min) (point) '(line-prefix ""))

- evaluate this form

Expected result:  No change in display
Actual result:  The formerly wrapped line is no longer wrapped but
displayed as if truncate-lines had been set (it is not set)

I suspect that the empty string causes the problem.  A nil line-prefix
property, or a non-empty string both work fine.

It is possible (likely?) that the same error also occurs with
wrap-prefix, but I have not tested this.



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
/Users/dominik/lib/git/emacs-new/etc/DEBUG.


In GNU Emacs 23.1.50.2 (i386-apple-darwin9.8.0, X toolkit, Xaw3d  
scroll bars)
  of 2009-11-20 on carsten-dominiks-macbook-pro.local
Windowing system distributor `The X.Org Foundation', version  
11.0.10402000
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: nil
   value of $XMODIFIERS: nil
   locale-coding-system: nil
   default enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
   show-paren-mode: t
   iswitchb-mode: t
   diff-auto-refine-mode: t
   tooltip-mode: t
   mouse-wheel-mode: t
   menu-bar-mode: t
   file-name-shadow-mode: t
   global-font-lock-mode: t
   font-lock-mode: t
   blink-cursor-mode: t
   global-auto-composition-mode: t
   auto-composition-mode: t
   auto-encryption-mode: t
   auto-compression-mode: t
   line-number-mode: t
   transient-mark-mode: t

Recent input:
? ? ? <tab> <down> <tab> <tab> <down> <tab> <tab> <tab>
C-x C-f $ o r g e l / t t t <backspace> <backspace>
e <tab> t t t . o r g <return> <return> <return> <return>
M-x f u n d a m <tab> <return> SPC <backspace> k j
f SPC a l k f j SPC l a k j f SPC a l k j f SPC a l
k j f SPC a l k j f h SPC a l k f j SPC a l k j f h
SPC a l k j f SPC a l k f j h SPC a l k f j h SPC a
l k j f h SPC a l k j f h SPC l a k j f SPC l a k j
f h SPC l a k j s f h SPC l a k j s f h SPC l k a j
s h f SPC l k j SPC <up> <up> <up> C-d C-d C-d <down>
<down> <return> ( a d d - t e x M-/ <escape> <tab>
t - p r o p e r t i e s SPC ( p o i n t - m i n ) SPC
( p o i n t ) SPC ' ( l i n e - p r e f i x SPC " "
) ) C-x C-e <up> <up> <down> <down> <up> <left> <down>
C-a C-SPC C-e <escape> w M-x s u b m i t - b u <tab>
<backspace> <tab> <backspace> <tab> <tab> <escape>
<tab> C-a C-k <tab> C-x o C-x o C-s - b u g C-s C-s
C-s C-s C-s C-s C-s C-s C-s C-s <help-echo> <down-mouse-1>
<mouse-2>

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Features:
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advice help-fns advice-preload derived org-footnote org-src org-list
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custom widget hashtable-print-readable backquote make-network-process
font-render-setting x-toolkit x multi-tty emacs)