Fix HTML entity ref breakage in &message8byte;

Thanks to yammaboo;
https://github.com/yammaboo/akahuku-firefox-sp/commit/7f9a1756e882d4c16efde97397dabc0a3f511118
Unescape message8byte for subdir for Save image

Thanks to yammaboo's report:
https://github.com/toshiakisp/akahuku-firefox-sp/pull/3