pf: reduce indentation

Early-return to reduce syncookie-related indentation.

No functional change.

MFC after: 1 week
Reviewed by: kp
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D41502

(cherry picked from commit 9642d948c08d80a41679f2eaab26e85a8b9e3080)