ike-init: Make sure we received a KE payload as initiator

On the responder, dc8b479eb8 ("ike-init: Fix key derivation if SA is
reset after IKE_INTERMEDIATE retransmits") fixed this.  But the initiator
still accepted IKE_INTERMEDIATE exchanges without KE payload.

Fixes: 0d49ddec2e ("ike-init: Add support for multiple key exchanges")

src/libcharon/sa/ikev2/tasks/ike_init.c

@@ -939,6 +939,7 @@ static void process_payloads_multi_ke(private_ike_init_t *this,
 	else
 	{
 		DBG1(DBG_IKE, "KE payload missing in message");
+		this->ke_failed = TRUE;
 	}
 }