Compiler warning in xdisp.c

[Po Lu]

Compiling master, I get:

In file included from composite.h:29,
                 from xdisp.c:441:
xdisp.c: In function ‘produce_stretch_glyph’:
dispextern.h:1891:10: warning: ‘face’ may be used uninitialized in this 
function [-Wmaybe-uninitialized]
 1891 |   return face_for_char (f, face, character, pos, object);
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
xdisp.c:30063:16: note: ‘face’ was declared here
30063 |   struct face *face;
      |                ^~~~

I'm not comfortable enough with that code to fix it.  The problem may
also exist in Emacs 28.  Thanks.