get_win32_message_utf8ness() Fix unlikely case

get_win32_message_utf8ness() Fix unlikely case

This function calls get_locale_string_utf8ness_i().  The API of that
function changed earlier in the 5.37 series, and this was not updated to
correspond.  It is calling it with category 0, which it thinks is an
out-of-bounds value, but it really is LC_ALL on Windows.  Thus it
actually probably doesn't lead to a failure, but it theoretically could
fail if LC_ALL is not uniform.

So it's better to not succeed just by accident.  This commit changes to
now use LC_CTYPE as the category.  And if someone has managed to compile
Windows without LC_CTYPE, it now just says that the input string isn't
UTF-8, as without LC_CTYPE, you've only got the C locale.