bug#68940: 29.2; Random crashes in face for char / font

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

> ../../emacs/src/lisp.h:1314: Emacs fatal error: assertion failed:
> FIXNUMP (a)
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Sorry, forgot to include the bt:

    frame #8: 0x000000010027c1f8 emacs`die(msg="FIXNUMP (a)", file="../../emacs/src/lisp.h", line=1314) at alloc.c:8062:3
    frame #9: 0x00000001003076b4 emacs`XFIXNUM(a=0x000000015b17036d) at lisp.h:1314:3
    frame #10: 0x0000000100307198 emacs`font_style_to_value(prop=FONT_WEIGHT_INDEX, val=0x000000015b17036d, noerror=true) at font.c:428:27
    frame #11: 0x0000000100310980 emacs`font_select_entity(f=0x000000015b172040, entities=0x0000000150ffef43, attrs=0x000000015ae06d60, pixel_size=12, c=9207) at font.c:3160:5
    frame #12: 0x00000001003104d0 emacs`font_find_for_lface(f=0x000000015b172040, attrs=0x000000015ae06d60, spec=0x000000015b085a15, c=9207) at font.c:3321:15
  * frame #13: 0x00000001003eba14 emacs`fontset_find_font(fontset=0x000000014b61c99d, c=9207, face=0x000000015ae06d60, charset_id=-1, fallback=false) at fontset.c:732:21
    frame #14: 0x00000001003e3490 emacs`fontset_font(fontset=0x000000012c209205, c=9207, face=0x000000015ae06d60, id=-1) at fontset.c:809:4
    frame #15: 0x00000001003e29a8 emacs`face_for_char(f=0x000000015b172040, face=0x000000015ae06d60, c=9207, pos=-1, object=0x0000000000000000) at fontset.c:1031:15
    frame #16: 0x000000010031995c emacs`FACE_FOR_CHAR(f=0x000000015b172040, face=0x000000015ae06d60, character=9207, pos=-1, object=0x0000000000000000) at dispextern.h:1950:10
    frame #17: 0x0000000100319658 emacs`Finternal_char_font(position=0x0000000000000000, ch=0x0000000000008fde) at font.c:4766:13

I will inspect the fontset, face, ... values in these frames later.