bug#42160: Issue with Man-kill and background process.

[Ergus via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

When using man with big manuals (like gcc). There is an error when
trying to quit with k just after opening the manual.
This seems to be related with the formating process that is running in
the background.

To reproduce the issue:

emacs -Q
M-x toggle-debug-on-error RET
M-x man RET
gcc RET
k

And I get this error message:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  Man-bgproc-filter(#<process man> "nt, there is no need to use address spaces like \"_...")


In GNU Emacs 28.0.50 (build 8, x86_64-pc-linux-gnu, X toolkit, cairo version 1.16.0, Xaw scroll bars)
 of 2020-06-30 built on ergus
Repository revision: 519e64f98140b984e10a9567017c7e5c4a81ff89
Repository branch: master
System Description: Debian GNU/Linux 10 (buster)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Debug on Error enabled globally
Invoking man gcc in the background
next-line: End of buffer [4 times]
Entering debugger...

Configured using:
 'configure --prefix=/home/ergus/.local/ --with-mailutils'

Configured features:
XPM JPEG TIFF GIF PNG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY
LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS
LUCID X11 XDBE XIM MODULES THREADS PDUMPER GMP

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

Major mode: Debugger

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
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec password-cache epa derived epg epg-config gnus-util
rmail rmail-loaddefs text-property-search time-date subr-x seq mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
cl-print debug backtrace help-mode find-func cl-seq imenu man easymenu
cl-loaddefs cl-lib ansi-color cus-start cus-load term/tmux term/xterm
xterm byte-opt gv bytecomp byte-compile cconv tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer 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 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 dbusbind inotify dynamic-setting system-font-setting
font-render-setting cairo x-toolkit x multi-tty make-network-process
emacs)

Memory information:
((conses 16 62396 7811)
 (symbols 48 7549 1)
 (strings 32 19068 1764)
 (string-bytes 1 597034)
 (vectors 16 8450)
 (vector-slots 8 93078 6052)
 (floats 8 32 570)
 (intervals 56 197 5)
 (buffers 992 11))