bug#15224: 24.3; newline does not move point to next line

[Nils Anders Danielsson <nad@cse.gu.se>]

Bug description
---------------

In some situations newline fails to move point to the next line, even
though the newly inserted line is blank.

To reproduce the bug, run the following command:

   emacs -Q --eval '(progn (fundamental-mode)

                           (modify-syntax-entry ?/  ". 124b")
                           (modify-syntax-entry ?*  ". 23")
                           (modify-syntax-entry ?\n "> b")

                           (setq comment-start "// ")

                           (auto-fill-mode 1)
                           (setq fill-column 9)
                           (setq comment-auto-fill-only-comments t)

                           (insert "/*  //x  x")
                           (newline))'

Expected result: A newline character should be inserted, and point
should move to the next line.

Actual result: A newline character is inserted, but point is not moved.
The *Messages* buffer contains the following message:

   comment-search-backward: Beginning of buffer

The definition of comment-search-backward contains the following
possibly relevant comment:

   ;; FIXME: If a comment-start appears inside a comment, we may erroneously
   ;; stop there.  This can be rather bad in general, but since
   ;; comment-search-backward is only used to find the comment-column (in
   ;; comment-set-column) and to find the comment-start string (via
   ;; comment-beginning) in indent-new-comment-line, it should be harmless.

Emacs configuration
-------------------

In GNU Emacs 24.3.1 (i686-pc-linux-gnu, GTK+ Version 3.6.4)
  of 2013-08-22 on meitnerium, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11303000
System Description:	Ubuntu 13.04

Configured using:
  `configure '--build' 'i686-linux-gnu' '--build' 'i686-linux-gnu'
  '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
  '--localstatedir=/var/lib' '--infodir=/usr/share/info'
  '--mandir=/usr/share/man' '--with-pop=yes'
  '--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.3/site-lisp:/usr/share/emacs/site-lisp'
  '--with-crt-dir=/usr/lib/i386-linux-gnu' '--with-x=yes'
  '--with-x-toolkit=gtk3' '--with-toolkit-scroll-bars'
  'build_alias=i686-linux-gnu' 'CFLAGS=-g -O2 -fstack-protector
  --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall'
  'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro'
  'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
   value of $LC_COLLATE: en_GB.UTF-8
   value of $LC_CTYPE: en_GB.UTF-8
   value of $LC_MESSAGES: en_GB.UTF-8
   value of $LC_MONETARY: sv_SE.UTF-8
   value of $LC_NUMERIC: sv_SE.UTF-8
   value of $LC_TIME: en_DK.UTF-8
   value of $LANG: en_GB.UTF-8
   locale-coding-system: utf-8-unix
   default enable-multibyte-characters: t

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util
mail-prsvr mail-utils help-mode easymenu time-date 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 dbusbind dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty emacs)

-- 
/NAD