crypto: clear out buffer after timing pbkdf algorithm

The 'out' buffer will hold a key derived from master
password, so it is best practice to clear this buffer
when no longer required.

At this time, the code isn't worrying about locking
buffers into RAM to prevent swapping sensitive data
to disk.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
