libpfctl: Don't pass stack garbage to free.

GCC 9 on stable/12 reports a -Wmaybe-uninitialized error for the call
to free in _pfctl_clear_states.

Reviewed by: mjg
Differential Revision: https://reviews.freebsd.org/D39198

(cherry picked from commit 48c519be0e8005aedc817b1bc1101ad32b67a67a)