pf: Handle errors returned by pf_killstates()

Happily this wasn't a real bug, because pf_killstates() never fails, but
we should check the return value anyway, in case it does ever start
returning errors.

Reported by: clang --analyze
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit fa96701c8abbc29aad7f8f8d6b823bd7f89c6c15)