pf: don't assert on address family in pf_addrcpy()

This gets called (through PF_ACPY()) on user supplied input, so we can't assume
that the address family must be AF_INET or AF_INET6.

Reported-by: syzbot+14dec0c55b8f512e271c@syzkaller.appspotmail.com
Sponsored by: Rubicon Communications, LLC ("Netgate")