EAP-SIM/AKA: Explicitly check for header to include Reserved field

EAP-SIM/AKA: Explicitly check for header to include Reserved field

This was previously checked as part of the eap_sim_parse_attr()
processing, but it is easier to review the code if there is an
additional explicit check for confirming that the Reserved field is
present since the pos variable is advanced beyond it.

Signed-off-by: Jouni Malinen <j@w1.fi>