bug#22356: 25.0.50; emacs-25 very crashy with ivy-mode

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 16 Jan 2016 10:22:53 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: 22356@debbugs.gnu.org
> 
>  > We weren't assigning space for glyphs of the margins in the
>  > mini-window.  Why? because of a single typo (or thinko?) more than 2
>  > years ago!  That typo made us think the mini-window has a very large
>  > total width in column units:
>  >
>  >    emacs -Q
>  >    M-: (window-total-width (minibuffer-window)) RET
>  >      => 672
>  >
>  > That preposterous value then defeated the logic in the display engine
>  > which decides how many glyphs are needed for displaying stuff in the
>  > margin, when margins are turned on in a window: that logic decided the
>  > correct number was zero.  And from there it's a quick path to writing
>  > beyond the allocated memory.
> 
> Thanks for fixing my silly oversight.

Stuff happens ;-)

> margin_glyphs_to_reserve is the only client of the total_cols slot
> of the minibuffer window, so there's no wonder that this bug
> remained undetected for such a long time.

margin_glyphs_to_reserve also had a flaw: it could return zero for
positive values of the MARGIN argument, which is clearly wrong.  I
fixed that with a follow-up patch, so now it always returns at least 1
column when the margin is non-zero.