pf: Fix an off-by-one in pf_handle_getcreators()

Reviewed by: kp
Reported by: Kevin Day <kevin@your.org>
MFC after: 1 week
Fixes: a7191e5d7b62 ("pf: add a way to list creator ids")
Differential Revision: https://reviews.freebsd.org/D54083

(cherry picked from commit 0795e9b53cf5be68955b49dce87f4800fdfd4a89)