ikev1: Ensure local IP is known as identity fallback during Main Mode

ikev1: Ensure local IP is known as identity fallback during Main Mode

We usually have a local IP already via ike_sa_t::resolve_hosts() before
build_i() is called but if that's not the case, it's more likely we have
one after we processed the first response (it might also have changed).
There is a potential chance we still don't have one if the socket API
doesn't provide us with the destination address of received messages,
but that seems not very likely nowadays.