pf: Improve DIOCADDRULE validation

We expect the addrwrap.p.dyn value to be set to NULL (and assert such),
but do not verify it on input.

Reported-by: syzbot+936a89182e7d8f927de1@syzkaller.appspotmail.com
Reviewed by: melifaro (previous version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24538