EAP-pwd: Clear identity string and temporary buffer explicitly

EAP-pwd: Clear identity string and temporary buffer explicitly

Use an explicit memset call to clear any configuration parameter and
dynamic data that contains private information like keys or identity.
This brings in an additional layer of protection by reducing the length
of time this type of private data is kept in memory.

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