bug#48408: BUGFIX: window-default-font-height: check for nil string

[martin rudalics <rudalics@gmx.at>]

 >> Sorry.  This function is missing from my current window.el here ...
 >
 > Really?  It is still there on today's master.  So I wonder what kind
 > of revolution do you have waiting in the wings...

Nothing special.  Just a function that takes its buffer's face remapping
into account.  But it lacks an alias and is apparently just as broken as
the old one, see below.

martin


(defun window-face-font-height (&optional window buffer-or-name face)
   "Return font height in pixels of FACE for BUFFER-OR-NAME in WINDOW.
WINDOW must be a live window and defaults to the selected one.
BUFFER-OR-NAME may be a buffer or the name of an existing buffer
and defaults to WINDOW's buffer.  FACE must specify a valid face
and defaults to the default face.

The return value accounts for any remapping of FACE's font (see
`face-remapping-alist') in BUFFER-OR-NAME.  If no such remapping
has been specified, return the height of the font of FACE for
WINDOW's frame.  If BUFFER-OR-NAME does not specify WINDOW's
buffer, calculate the return value with WINDOW's buffer
temporarily set to the buffer specified by BUFFER-OR-NAME."
   (let* ((window (window-normalize-window window t))
          (frame (window-frame window)))
     (if (display-multi-font-p frame)
         (let* ((buffer (if buffer-or-name
                            (window-normalize-buffer buffer-or-name)
                          (window-buffer window)))
                (face (or face 'default))
                (font (if (eq window (selected-window))
                          (window--buffer-face-font buffer face)
                        (with-selected-window window
                          (window--buffer-face-font buffer face))))
                (frame-font (frame-parameter frame 'font)))
	  ;; Avoid calling `font-info' if the font whose info we want is
	  ;; the same as the corresponding frame's default font since
	  ;; `font-info' is expensive for some fonts, see bug #14838.
           (if (and font (not (string-equal font frame-font)))
               (aref (font-info font frame) 3)
             (frame-char-height frame)))
       (frame-char-height frame))))