pf: fix possible pd->pcksum NULL deref

Ensure that we always populate pcksum, not just for TCP/UDP/SCTP/ICMP.
We may end up using it (through pf_change_ap()) if we're doing NAT, so
ensure it's available even if it's sometimes just a dummy variable.

Reported-by: syzbot+3e73a7cba8cb6cd46f90@syzkaller.appspotmail.com
MFC after: 1 week
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