cmp: initialize limit to avoid stack garbage limits

Approved by: re (gjb)
Pointy hat: kevans
Fixes: 4e380e847460 ("cmp: add -n, --bytes to limit number of bytes [...]")
Sponsored by: Klara, Inc.

(cherry picked from commit f1f7f31366e5830f4f5b96c6394a0b91475744f4)
(cherry picked from commit aba2dc46dfa51cf9e1b1987ca4e91f703ae7e368)