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

[martin rudalics]

> 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.  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.

martin