This converts tls_connection_handshake(),
tls_connection_server_handshake(), tls_connection_encrypt(), and
tls_connection_decrypt() to use struct wpa_buf to allow higher layer
code to be cleaned up with consistent struct wpabuf use.
When the TLS handshake had been completed earlier by the server in case of
abbreviated handshake, the output buffer length was left uninitialized. It
must be initialized to zero in this case. This code is used by EAP-FAST
server and the uninitialized length could have caused it to try to send a
very large frame (though, this would be terminated by the 50 roundtrip EAP
limit). This broke EAP-FAST server code in some cases when PAC was used to
establish the tunnel.
