bug#17283: 24.3.50; rmail-quit tries to work with deleted rmail-summary-buffer

[Ken Olum <kdo@cosmos.phy.tufts.edu>]

If you have an rmail-summary buffer that you don't want, a
logical-seeming thing to do would be just to kill the buffer.  But then
when you quit rmail, rmail-quit tries to do various operations to the
deleted buffer, and you get "Selecting deleted buffer".

To reproduce:

Run rmail.
Type h to get summary buffer
Kill summary buffer
Type q to rmail
Get error: "Selecting deleted buffer"

This could be fixed by replacing (when rmail-summary-buffer ...) by
(when (rmail-summary-exists) ...) in rmail-quit.

                                        Ken

In GNU Emacs 24.3.50.4 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
 of 2014-01-06 on cosmos
Bzr revision: 115759 kdo@cosmos.phy.tufts.edu-20131227194350-g3o5h6bnyg8yntf2
Windowing system distributor `The Cygwin/X Project', version 11.0.10903000
System Description:	Ubuntu 12.04.4 LTS

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

Major mode: Messages

Minor modes in effect:
  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
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(pp rmailsum rmailmm rmail help-mode 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 time-date 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 inotify
dynamic-setting font-render-setting x-toolkit x multi-tty emacs)