bug#41705: 28.0.50; minibuffer completion of ".../*/*" shouldn't be "...//"

[Pip Cet]

Recipe:

./emacs -Q
enter: C-x C-f * / * <tab>

expected result: a list of sub-subdirectories of the emacs src dir. A
"Find file: .../emacs/src/*/" prompt.

actual result: prompt says "Find file: .../emacs/src//", further tab
completion uses the root directory.

I'd noticed this for a while now, but thought it was a local
configuration issue. It happens in emacs -Q, too, though.

For master, we should fix minibuffer.el properly, but I'm not sure what
to do on emacs-27. It's a regression (from Emacs 26) that might annoy
many users of tab completion in the minibuffer.