Add eap-tls key derivation in the examples.

Add support for eap-tls key derivation functionality,
in `ssl_client2` and `ssl_server2` reference applications.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 6a6ed0e..620adf9 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1270,7 +1270,7 @@
     {
         ssl->conf->f_export_keys_ext( ssl->conf->p_export_keys,
                                       session->master, keyblk,
-                                      mac_key_len, transform->keylen,
+                                      mac_key_len, keylen,
                                       iv_copy_len, handshake->tls_prf,
                                       handshake->randbytes + 32,
                                       handshake->randbytes );