[tls] Ensure received data list is initialised before calling tls_free()

A failure in tls_generate_random() will result in a call to ref_put()
before the received data list has been initialised, which will cause
free_tls() to attempt to traverse an uninitialised list.

Fix by ensuring that all fields referenced by free_tls() are
initialised before any of the potential failure paths.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tls.c

 	tls_clear_cipher ( tls, &tls->rx_cipherspec );
 	tls_clear_cipher ( tls, &tls->rx_cipherspec_pending );
 	tls->client_random.gmt_unix_time = time ( NULL );
+	iob_populate ( &tls->rx_header_iobuf, &tls->rx_header, 0,
+		       sizeof ( tls->rx_header ) );
+	INIT_LIST_HEAD ( &tls->rx_data );
 	if ( ( rc = tls_generate_random ( tls, &tls->client_random.random,
 			  ( sizeof ( tls->client_random.random ) ) ) ) != 0 ) {
 		goto err_random;
 		      ( sizeof ( tls->pre_master_secret.random ) ) ) ) != 0 ) {
 		goto err_random;
 	}
-	iob_populate ( &tls->rx_header_iobuf, &tls->rx_header, 0,
-		       sizeof ( tls->rx_header ) );
-	INIT_LIST_HEAD ( &tls->rx_data );
 
 	/* Start negotiation */
 	tls_restart ( tls );