WPA: Explicitly clear the buffer used for decrypting Key Data

WPA: Explicitly clear the buffer used for decrypting Key Data

When AES-WRAP was used to protect the EAPOL-Key Key Data field, this was
decrypted using a temporary heap buffer with aes_unwrap(). That buffer
was not explicitly cleared, so it was possible for the group keys to
remain in memory unnecessarily until the allocated area was reused.
Clean this up by clearing the temporary allocation explicitly before
freeing it.

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