bug#25767: 25.1.91; abbrev-prefix-mark expansion inserts newline

[Allen Li]

Expanding an abbrev while using abbrev-prefix-mark inserts a newline.  Probably
this is best explained through a repro:

1. Start emacs -Q -nw
2. Go to scratch buffer
3. Define an abbrev from f to food: (type food, C-x a l f RET)
4. M-x abbrev-mode
5. Move point so it's at the asterisk:

  ;; This buffer is for text that is not saved, and for Lisp evaluation.
  ;; To create a file, visit it with C-x C-f and enter text in its buffer.

  *

  food

6. Type s M-' f SPC

Expected:

  ;; This buffer is for text that is not saved, and for Lisp evaluation.
  ;; To create a file, visit it with C-x C-f and enter text in its buffer.

  sfood *

  food

Actual:

  ;; This buffer is for text that is not saved, and for Lisp evaluation.
  ;; To create a file, visit it with C-x C-f and enter text in its buffer.

  sfood
   *

  food

In GNU Emacs 25.1.91.1 (x86_64-pc-linux-gnu, GTK+ Version 3.10.8),
modified by Debian
System Description: Ubuntu 14.04 LTS

Configured using:
 'configure --build x86_64-linux-gnu --build x86_64-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/google-emacs:/etc/emacs:/usr/local/share/emacs/25.1.91+gg1+2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.1.91+gg1+2/site-lisp:/usr/share/emacs/site-lisp
 --with-crt-dir=/usr/lib/x86_64-linux-gnu --disable-build-details
 --disable-silent-rules --with-modules GOOGLE_VERSION=25.1.91+gg1+2
 --with-x=yes --with-x-toolkit=gtk3 --with-toolkit-scroll-bars
 build_alias=x86_64-linux-gnu 'CFLAGS=-g -O2 -fstack-protector
 --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall'
 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro
 -Wl,-fuse-ld=gold,--export-dynamic-symbol=__google_auxv'
 'CPPFLAGS=-D_FORTIFY_SOURCE=2 -DGOOGLE_EMACS_DEFINE_AUXV''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-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
  transient-mark-mode: t
  abbrev-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Abbrev mode enabled in current buffer
next-line: End of buffer [3 times]

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec password-cache epg epg-config gnus-util mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util help-fns help-mode easymenu
cl-loaddefs pcase cl-lib mail-prsvr mail-utils term/xterm xterm
time-date mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel x-win term/common-win x-dnd tool-bar dnd fontset
image regexp-opt fringe tabulated-list newcomment elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow timer select scroll-bar
mouse jit-lock font-lock syntax facemenu font-core frame cl-generic cham
georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote dbusbind inotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 91659 6500)
 (symbols 48 20147 0)
 (miscs 40 42 146)
 (strings 32 14851 4772)
 (string-bytes 1 422197)
 (vectors 16 9899)
 (vector-slots 8 393090 9637)
 (floats 8 169 482)
 (intervals 56 254 0)
 (buffers 976 19)
 (heap 1024 8591 1066))