EAP-pwd peer: Fix error path for unexpected Confirm message

EAP-pwd peer: Fix error path for unexpected Confirm message

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>