bound check utf8 characters

bound check utf8 characters

since we load the fontmap from psf1 font files, we only have characters
for a maximum codepoint of 0xFFFF, so we save the char in an
unsigned short (16bit) but we decode up to 6 bytes of utf8
so we have to correctly bound check the assignment, else we can get
garbled characters on the terminal

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>