pf: 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
Obtained from: OpenBSD, sashan <sashan@openbsd.org>, 38bfd041cb
Sponsored by: Rubicon Communications, LLC ("Netgate")