If the Confirm message is received from the server before the Identity
exchange has been completed, the group has not yet been determined and
data->grp is NULL. The error path in eap_pwd_perform_confirm_exchange()
did not take this corner case into account and could end up
dereferencing a NULL pointer and terminating the process if invalid
message sequence is received. (CVE-2015-5316)
Signed-off-by: Jouni Malinen <j@w1.fi>