The idea of mapping non-existen characters to space (0x20) was nice alone,
but those maps also used as backward maps for Paste, so space becomes mapped
to last non-existen character on Paste as result.

Fix it by mapping non-existen characters to another non-existen one, i.e. to
0x00, so unused 0x00 can be backward-mapped to some junk without real harm.

Pointed by: Alexander Kabaev <ak03@gte.com>