overwrite icmp packet type-specific nextptr field only when we're changing it.  fixes an icmp to icmpv6 translation regression found by todd and sthen.
overwrite icmp packet type-specific nextptr field only when we're
changing it.  fixes an icmp to icmpv6 translation regression found
by todd and sthen.

ok henning sthen

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars