isearch-ring-advance and isearch-ring-retreat stop after first usage

[Christoph Conrad <christoph.conrad@gmx.de>]

Tested with emacs -q --no-site-file

Starting with C-s, and then M-p or M-n two times => Message "Text ist
read-only" and incremental search stops.

In GNU Emacs 22.0.95 (i386-msvc-nt5.1.2600)
  of 2007-03-05 on CLEOPATRA
X server distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-msvc (13.10)'

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: de
   locale-coding-system: cp1252
   default-enable-multibyte-characters: t

Major mode: C++/l

Minor modes in effect:
   hi-lock-mode: t
   partial-completion-mode: t
   erc-track-mode: t
   minibuffer-electric-default-mode: t
   which-function-mode: t
   outline-minor-mode: t
   auto-image-file-mode: t
   pc-selection-mode: t
   recentf-mode: t
   shell-dirtrack-mode: t
   show-paren-mode: t
   encoded-kbd-mode: t
   tooltip-mode: t
   mouse-wheel-mode: t
   file-name-shadow-mode: t
   global-font-lock-mode: t
   font-lock-mode: t
   unify-8859-on-encoding-mode: t
   utf-translate-cjk-mode: t
   auto-compression-mode: t
   line-number-mode: t
   transient-mark-mode: t
   abbrev-mode: t

Recent input:
M-x <up> <return> <up> <return> <down> <up> <C-right>
<C-right> C-s C-w C-w C-w C-s C-s C-s C-s C-s C-s <home>
<M-delete> C-s M-p M-p M-p M-p M-p M-p M-p <up> <up>
<up> C-s RET <up> <up> <up> <up> <home> <C-backspace>
M-x r e p o r t <tab> e m <tab> <return> I s e a r
c h SPC a n d SPC <C-M-up> C-g <C-M-up> C-g <C-backspace>
a a <C-M-up> <C-M-up> M-x <up> <up> <return> <up> <up>
<return> <C-right> <C-right> C-s C-s C-w C-w <right>
<left> C-s C-w C-w C-w C-s C-s <home> <M-delete> M-x
<up> <up> <return>

Recent messages:
Updating buffer list...done
Commands: m, u, t, RET, g, k, S, D, Q; q to quit; h for help
Loading emacsbug...done
c-forward-sws: Wrong type argument: stringp, nil [2 times]
Quit [10 times]
Updating buffer list...done
Commands: m, u, t, RET, g, k, S, D, Q; q to quit; h for help
Mark set
Mark saved where search started
c-forward-sws: Wrong type argument: stringp, nil [5 times]