Update ustar creation sanity check to use WCS path on Windows (#2230)

Update ustar creation sanity check to use WCS path on Windows (#2230)

On Windows, the MBS pathname might be null if the string was set with a
WCS that can't be represented by the current locale. This is handled
properly by the rest of the code, but there's a sanity check that does
not make the proper distinction.

Note: this is a partial cherry-pick from
https://github.com/libarchive/libarchive/pull/2095, which I'm going to
go through and break into smaller pieces in hopes of getting some things
in while discussion of other things can continue.