fix zero division found by syzkaller. The sanity checks in pf(4) ioctls

fix zero division found by syzkaller. The sanity checks in pf(4) ioctls
are not powerful enough to detect invalid port ranges (or even invalid
rules). syzkaller does not use pfctl(8), it uses ioctl(2) to pass some
random chunk of memory as a rule to pf(4). Fix adds explicit check
for 0 divider to pf_get_transaddr(). It should make syzkaller happy
without disturbing anyone else.

OK gnezdo@

Reported-by: syzbot+d1f00da48fa717e171f3@syzkaller.appspotmail.com