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")