ITS#7992 Tighter utf8_to_utf16(), fix errcodes
ITS#7992 Tighter utf8_to_utf16(), fix errcodes

The 0xFFFD check seems due to misleading MultiByteToWideChar() doc.
Bad UTF-8 gives 0xFFFD in the output string, not the return value.

bobek-balinek/BlueJet