Fix regression in LEAP

Commit 458cb3019108b6cb8c0c1cab94ae6ebf244eda27 broke LEAP since it
rejects EAP-Success packet that is used within LEAP and this frame does
not have a payload. Fix LEAP by relaxing the generic EAP packet
validation if LEAP has been negotiated.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>