bug#23091: 25.0.92; :box face hang regression

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Tue, 22 Mar 2016 14:38:21 +0000
> 
> While using the monokai theme and helm, a hang can be triggered with a
> setq header-line-format in a specific shape. The below is a repro from
> an emacs -Q. This does not repro in Emacs 24 or terminal emacs.
> I am on OS X.
> The hang does not repro if there is no :box in the face defined below.

Fixed.

(It was sheer luck that this worked in Emacs 24, the code which
handles this situation made wrong assumptions.)

Thanks.