pf: Fix build if INVARIANTS is not set

r340061 included a number of assertions pf_frent_remove(), but these assertions
were the only use of the 'prev' variable. As a result builds without
INVARIANTS had an unused variable, and failed.

Reported by: vangyzen@

(cherry picked from commit 58ef854f8b05508f41aff3bdaf1564c8dd4c1d4f)