pubkey-authenticator: Avoid conflict with config switch based on EAP-Identity

The referenced commit ignored that INVALID_ARG was returned by this
authenticator if an unsupported signature scheme is encountered.  This
caused a crash in find_alternative_eap_cfg() as no EAP identity is
stored in the current auth config.

Since we don't distinguish the situation outside of the authenticator,
we can just return FAILED.

Closes strongswan/strongswan#2979

Fixes: 2f2e4abe3c ("ikev2: Add support to switch peer configs based on EAP-Identities")

src/libcharon/sa/ikev2/authenticators/pubkey_authenticator.c

@@ -631,7 +631,7 @@ METHOD(authenticator_t, process, status_t,
 			DBG1(DBG_IKE, "%N authentication %s", auth_method_names,
 				 auth_method, reason);
 			signature_params_destroy(params);
-			return INVALID_ARG;
+			return FAILED;
 	}
 	id = this->ike_sa->get_other_id(this->ike_sa);
 	if (!get_auth_octets_scheme(this, TRUE, id, this->ppk, &octets, &params))