I just noticed that one can resize mini windows down to one pixel
which is maybe not what users want.  To reproduce with emacs -Q simply
type C-h f and drag the modeline of the only window down with the
mouse.  With some insistence you can make the minibuffer window almost
disappear.

A trivial patch is attached.  I think it should go to the release
branch but maybe the problem is too cosmetic for that.

Thanks, martin