vt: selection: handle storing of characters above U+FFFF

Those above U+10FFFF get replaced with U+FFFD.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>