bug#28978: 26.0; Regression: separate, dedicated `*Completions*' frame no longer has parameter `minibuffer'

[Drew Adams <drew.adams@oracle.com>]

>  > Before your change, a separate *Completions* frame didn't
>  > have a minibuffer-window value for its frame parameter
>  > `minibuffer' - it used another frame's minibuffer.  Before
>  > your change checking that parameter was a reasonable way
>  > to check whether the frame had an active minibuffer window.
> 
> Your check does not do that.  It tries to check whether this-frame has
> no minibuffer window and no minibuffer window is active.  Given the
> Emacs 25 manual which says about the 'minibuffer' parameter
> 
>       Whether this frame has its own minibuffer.  The value `t' means
>       yes, `nil' means no, `only' means this frame is just a minibuffer.
>       If the value is a minibuffer window (in some other frame), the
>       frame uses that minibuffer.

Yes, the doc has said that since at least Emacs 22.  But it
has not been true until now, AFAICT.  In the case in point,
my *Completions* frame uses my standalone minibuffer frame,
but parameter `minibuffer' in *Completions* remained nil,
even during completion (i.e., during use of the active
minibuffer).