nitcc: end-of-stream does not equate `'\0'`

It was a basic WIP stuff that remained.
`Any` and other classes of character can legally accept `'\0'` thus looping
infinitely.

Reported-by: Samuel Lambert <tweakedsam@gmail.com>
Signed-off-by: Jean Privat <jean@pryen.org>
nitcc/tests: add lexer-shortest.sablecc

Signed-off-by: Samuel Lambert <tweakedsam@gmail.com>
Signed-off-by: Jean Privat <jean@pryen.org>