bug#33510: 26.1.90; M-x man crashes if invoking man fails

[Richard Copley <rcopley@gmail.com>]

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If invoking 'man' fails, M-x man crashes, with errors:
  error in process sentinel: read-event: Buffer is read-only: #<buffer *Man
xyzzy*>
  error in process sentinel: Buffer is read-only: #<buffer *Man xyzzy*>

To reproduce from 'emacs -Q':
M-x man RET
xyzzy RET

In GNU Emacs 26.1.90 (build 1, x86_64-w64-mingw32)
 of 2018-11-22 built on [redacted]
Repository revision: 93242b14769ed40ae58e89d0ea45df8872f59869
Windowing system distributor 'Microsoft Corp.', version 6.1.7601
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Invoking man xyzzy in the background
error in process sentinel: read-event: Buffer is read-only: #<buffer *Man
xyzzy*>
error in process sentinel: Buffer is read-only: #<buffer *Man xyzzy*>

Configured using:
 'configure --config-cache --with-modules --without-pop --without-dbus
 --without-gconf --without-gsettings 'CFLAGS=-O3 -ggdb3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS MODULES THREADS LCMS2

Important settings:
  value of $LANG: ENG
  locale-coding-system: cp1252

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  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
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv dired dired-loaddefs format-spec rfc822 mml
mml-sec password-cache epa derived epg epg-config gnus-util rmail
rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils imenu man easymenu cl-loaddefs cl-lib
ansi-color elec-pair time-date mule-util tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel dos-w32 ls-lisp disp-table
term/w32-win w32-win w32-vars term/common-win tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors 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 composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray 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 threads w32notify w32 lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 99435 9461)
 (symbols 48 20447 1)
 (miscs 40 40 95)
 (strings 32 30599 928)
 (string-bytes 1 792435)
 (vectors 16 14455)
 (vector-slots 8 505151 11586)
 (floats 8 56 218)
 (intervals 56 239 8)
 (buffers 992 12))

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