While this doesn't really create any problems it is not 100% correct to
accept such messages because, of course, the sender of an IKE_SA_INIT
request is always the original initiator of an IKE_SA.
We currently don't check the flag later, so we wouldn't notice if the
peer doesn't set it in later messages (ike_sa_id_t.equals doesn't
compare it anymore since we added support for IKEv1, in particular since
17ec1c74de).