dired, ido-mode etc and SIGFPE in font code

[Herbert Euler]

I have some files in my home directory, whose names contains GBK
characters.  I found if I evaluate the below form after Emacs is
started:

  (prefer-coding-system 'utf-8)

Opening my home directory with dired and scrolling the dired buffer up
to reveal those files will cause a SIGFPE signal delivered and Emacs
hangs there.  This is what I get from gdb:

(gdb) r -Q
Starting program: /data/emacs/src/emacs -Q
[Thread debugging using libthread_db enabled]
[New Thread 0xb70f9a40 (LWP 21640)]
                                        (evaluating "(prefer-coding-system 
'utf-8)")
Program received signal SIGFPE, Arithmetic exception.
[Switching to Thread 0xb70f9a40 (LWP 21640)]
0x08271f50 in xfont_open (f=0x853bd80, entity=140224556, pixel_size=18) at 
xfont.c:616
616               font->average_width = width / n;
(gdb) p n
$1 = 0
(gdb)

I don't know if this is a dired bug or a font code bug.  Btw, ido-mode
has a similar problem as dired, only that it doesn't cause Emacs to
hang.

Regards,
Guanpeng Xu
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