bug#30721: 24.5; sh-mode sexp search with heredoc

[Ole Tange]

Generate a file with this:

    (echo '#!/bin/bash'
        echo 'myfunc() {'
            echo "cat <<'z'"
                seq 135
                    echo zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
                        echo z
                            echo '}') >out

Open it in Emacs. When pressing `C-M-f` I get the error:

    Scan error: "Containing expression ends prematurely", 515, 515


In GNU Emacs 24.5.1 (x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2017-09-20 on lcy01-07, modified by Debian
System Description:    Ubuntu 16.04.2 LTS

Configured using:
 `configure --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/emacs24:/etc/emacs:/usr/local/share/emacs/24.5/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.5/site-lisp:/usr/share/emacs/site-lisp
 --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/emacs24:/etc/emacs:/usr/local/share/emacs/24.5/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.5/site-lisp:/usr/share/emacs/site-lisp
 --with-x=yes --with-x-toolkit=gtk3 --with-toolkit-scroll-bars
 'CFLAGS=-g -O2 -fstack-protector-strong -Wformat
 -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro''

Important settings:
  value of $LC_TIME: C
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Shell-script

Minor modes in effect:
  sh-electric-here-document-mode: t
  tooltip-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

Recent messages:
Loading /etc/emacs/site-start.d/50autoconf.el (source)...done
Loading /etc/emacs/site-start.d/50cmake-data.el (source)...done
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...
Loading debian-ispell...done
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...done
Loading /etc/emacs/site-start.d/50python-docutils.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Setting up indent for shell type bash
Indentation variables are now local.
Indentation setup for shell type bash

Load-path shadows:
/usr/share/emacs/24.5/site-lisp/debian-startup hides
/usr/share/emacs/site-lisp/debian-startup
/usr/share/emacs24/site-lisp/cmake-data/cmake-mode hides
/usr/share/emacs/site-lisp/cmake-mode
/usr/share/emacs/site-lisp/rst hides /usr/share/emacs/24.5/lisp/textmodes/rst

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
easymenu mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util help-fns mail-prsvr mail-utils sh-script smie executable xterm
time-date wombat-theme tooltip electric uniquify 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 prog-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 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 make-network-process dbusbind
gfilenotify dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty emacs)

Memory information:
((conses 16 83373 8015)
 (symbols 48 18168 0)
 (miscs 40 34 124)
 (strings 32 11772 6456)
 (string-bytes 1 303730)
 (vectors 16 14158)
 (vector-slots 8 1122421 231186)
 (floats 8 65 233)
 (intervals 56 216 5)
 (buffers 960 12)
 (heap 1024 19046 1391))