fix write certificate fail

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_client.c b/library/ssl_tls13_client.c
index db9b762..cdb41f0 100644
--- a/library/ssl_tls13_client.c
+++ b/library/ssl_tls13_client.c
@@ -1970,8 +1970,13 @@
 {
     int ret;
 
-    mbedtls_ssl_set_outbound_transform( ssl, ssl->handshake->transform_handshake );
-
+    if( !ssl->handshake->client_auth )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1,
+                  ( "Switch to handshake traffic keys for outbound traffic" ) );
+        mbedtls_ssl_set_outbound_transform( ssl,
+                                        ssl->handshake->transform_handshake );
+    }
     ret = mbedtls_ssl_tls13_write_finished_message( ssl );
     if( ret != 0 )
         return( ret );