bug#40702: 28.0.50; (what-cursor-position) barfs on non-ASCII char

[Dima Kogan]

Hi. I'm using a very recent build of emacs from git. I see this:

1. emacs -Q
   Fresh emacs. Opens in the *scratch* buffer

2. C-x 8 ' e
   i.e. insert some non-ASCII character. Opening any buffer with such
   characters works too

3. Left
   Move the point to this character

4. C-x =
   (what-cursor-position) to ask emacs to tell us about this character.
   I see this:

   cl--assertion-failed: Assertion failed: (not (multibyte-string-p str))

Thanks!