ike-init: Make DH group reuse optional to test INVALID_KE_PAYLOAD handling

This is currently not an issue for CHILD_SA rekeying tests as these only
check rekeyings of the CHILD_SA created with the IKE_SA, i.e. there is
no previous DH group to reuse.

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

@@ -534,7 +534,8 @@ METHOD(task_t, build_i, status_t,
 	/* if we are retrying after an INVALID_KE_PAYLOAD we already have one */
 	if (!this->dh)
 	{
-		if (this->old_sa)
+		if (this->old_sa && lib->settings->get_bool(lib->settings,
+								"%s.prefer_previous_dh_group", TRUE, lib->ns))
 		{	/* reuse the DH group we used for the old IKE_SA when rekeying */
 			proposal_t *proposal;
 			uint16_t dh_group;