daemon/session2: try to avoid triggering Coverity

daemon/session2: try to avoid triggering Coverity

Strictly speaking, the .ip member is on 16 bytes long,
so it doesn't suffice in case it's an IPv6.
Practically it doesn't matter, as there's the union containing all this.

Either way, I think it's cleaner to copy to the whole union.
Also we don't support it in this case,
but theoretically e.g. an AF_UNIX could happen,
so let's be defensive and guard by kr_require().