pf: Make sure that pfi_update_status() always zeros counters

pfi_update_status() can return early if the status interface doesn't
exist. But in this case pf_getstatus() was copying uninitialized stack
memory into the output nvlist.

Reported by: Jenkins (KMSAN job)
Reviewed by: kp
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35659

(cherry picked from commit 333670372ffe2f1c562e1088a7779eb709006ba2)