tls-peer: Return INVALID_STATE after changing TLS 1.3 keys

tls-peer: Return INVALID_STATE after changing TLS 1.3 keys

Even though we return from build(), we are not actually sending a response,
so we can't return NEED_MORE (would send an invalid ClientHello message) and
if we return SUCCESS, the EAP layer treats this as failure (there is a comment
in eap_authenticator_t about client methods never returning SUCCESS from
process()).  Instead we return INVALID_STATE, which allows tls_t.build() to
exit from the build() loop immediately and send the already generated Finished
message.