Printing to STDERR when %d is "Cannot access memory ..."

[Keith David Bershatsky]

I am working on crosshairs (17684) and multiple fake cursors (22873) and 
created a little problem for myself in that a character not recognized by the 
font (U0001D4D5) [aka a pretty letter f] on a line all by itself does not have 
a font->space_width and causes Emacs to crash when printing said value to 
STDERR.

GDB returns "Cannot access memory at address 0xa0" when printing the value of 
font->space_width.

This came about by using the following setting for a Form Feed character:

(setq buffer-display-table (make-display-table))
(aset buffer-display-table
      ?\014
      (vector (make-glyph-code ?\U0001D4D5 'font-lock-warning-face)))

I am by passing the problem by not using ?\U0001D4D5.  However, I would like to 
come up with programmatic solution to avoid crashing Emacs when printing values 
to STDERR.

I tried:

fprintf (stderr, "(%d)", (font->space_width == NULL
                          ? 0
                          : font->space_width);

However, that didn't fix the problem.

Thanks,

Keith