bug#31192: 27.0.50; artist-mode throws a wrong type argument

[Tino Calancha]

X-Debbugs-CC: Lars Ingebrigtsen <larsi@gnus.org>

After commit '(artist-mode): Warn about proportional fonts'
(36e5673753b985dee06b255be237dce9e96b0eb7)

emacs -batch -eval "(artist-mode)"
throws
Wrong type argument: font, unspecified

That causes two ibuffer-tests.el to fail:
(ibuffer-filter-inclusion-7, ibuffer-filter-inclusion-8)

--8<-----------------------------cut here---------------start------------->8---
commit 04f528b864490f22e773d185f2a4980b29306eb0
Author: Tino Calancha <tino.calancha@gmail.com>
Date:   Tue Apr 17 21:34:25 2018 +0900

    * lisp/textmodes/artist.el (artist-mode): Ensure we have a font

diff --git a/lisp/textmodes/artist.el b/lisp/textmodes/artist.el
index 7c261f8d2d..e9ae6a4ce9 100644
--- a/lisp/textmodes/artist.el
+++ b/lisp/textmodes/artist.el
@@ -1402,8 +1402,9 @@ artist-mode
        (t
         ;; Turn mode on
         (artist-mode-init)
-         (unless (font-get (face-attribute 'default :font) :spacing)
-           (message "The default font isn't monospaced, so the drawings in 
this buffer may look odd")))))
+         (let ((font (face-attribute 'default :font)))
+           (when (and (fontp font) (not (font-get font :spacing)))
+             (message "The default font isn't monospaced, so the drawings in 
this buffer may look odd"))))))
 
 ;; Init and exit
 (defun artist-mode-init ()
--8<-----------------------------cut here---------------end--------------->8---
In GNU Emacs 27.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2018-04-17
Repository revision: 003071346cdc914f28a8e8b3a8bf0c8f5f3b52dc