bug#50519: 28.0.50; describe-fontset bug when indent-tabs-mode is nil

["Kévin Le Gouguec" <kevin.legouguec@gmail.com>]

Hi,

Unless I'm bisecting this wrong, 2021-08-20 "Fix printing of C-@ in
`describe-fontset'" (ac2cecbd83) introduced a regression when
indent-tabs-mode is nil:

$ emacs -Q
M-: (custom-set-variables '(indent-tabs-mode nil))
M-x describe-fontset RET fontset-default RET

⇒ print-fontset-element: Args out of range: #<buffer *Help*>, 1465, 1466

IIUC setting indent-tabs-mode to nil changes describe-vector's output
(since it eventually calls Findent_to), which I guess fools
print-fontset-element?

I haven't taken the time to fully understand how print-fontset-element
and mule--kbd-at operate, so I don't know if the correct fix would be
tweaking some regexp(s) or let-binding indent-tabs-mode to t somewhere.


In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.30, cairo version 1.16.0)
 of 2021-08-23 built on amdahl30
Repository revision: 00edc8329a6277f2e5b5204efbe503e2b7957006
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: openSUSE Tumbleweed

Configured using:
 'configure --with-xwidgets --with-cairo --with-gconf'

Configured features:
ACL CAIRO DBUS FREETYPE GCONF GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ
JPEG JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM XWIDGETS GTK3 ZLIB

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