bug#38191: incorrect text properties in result of `format' with multibyte(?) characters

[Paul Pogonyshev]

"Multibyte" is a guess, I don't really know the underlying reason.

Examples:

(format (propertize "`foo' %s bar" 'face 'bold) "xxx")
  => #("`foo' xxx bar" 0 13 (face bold))

(format (propertize "‘foo’ %s bar" 'face 'bold) "xxx")
  => #("‘foo’ xxx bar" 0 10 (face bold))

Length of the string is the same in both cases. In the first example
the face is correctly applied to the whole string, in the second
example 3 last characters incorrectly lack a face.

This is a regression, it used to work correctly before, but I don't
know when it became broken.

Tested on: GNU Emacs 27.0.50 (build 3, x86_64-pc-linux-gnu, GTK+
Version 2.24.32) of 2019-11-13

Paul