Use a separate error case handler for eap_pax_mac() failures and memcmp
to avoid wpa_hexdump() calls for the (mainly theoretical) local error
cases in deriving the MAC.
Fixes: b3c2b5d9f7d8 ("EAP-PAX server: Check hash function results")
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>