libpfctl: allow pfctl_free_status(NULL)

Mimic free() and friends, and allow free()ing of NULL.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D41648

(cherry picked from commit 0b01878fd00b128ce3dead119b37781048744d39)