scanner: allow for a zero terminated string as keymap

scanner: allow for a zero terminated string as keymap

As the documentation for xkb_keymap_new_from_buffer() states, the "input string
does not have to be zero-terminated". The actual implementation however failed
with "unrecognized token/syntax error" when it encountered a null byte.

Fix this by allowing a null byte at the last position of the buffer. Anything
else is likely a client error anyway.

Fixes #307