bug#38771: Allow face-attribute to be 'reset, (in addition the usual 'unspecified and valid-values)

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: deego3@gmail.com,  38771@debbugs.gnu.org
> Date: Sun, 03 Jul 2022 11:57:25 +0200
> 
> In file included from xfaces.c:226:
> xfaces.c: In function ‘Finternal_set_lisp_face_attribute’:
> lisp.h:337:26: warning: ‘tmp’ may be used uninitialized in this function 
> [-Wmaybe-uninitialized]
>   337 | #  define lisp_h_XLI(o) ((EMACS_INT) (o))
>       |                          ^
> xfaces.c:3539:23: note: ‘tmp’ was declared here
>  3539 |           Lisp_Object tmp;
>       |                       ^~~

Oops.  Should be fixed now.