tls13: guards transform negotiate

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index e4d50db..c9ef8e4 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -5097,9 +5097,12 @@
      * data.
      */
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "switching to new transform spec for inbound data" ) );
+#if defined(MBEDTLS_SSL_PROTO_TLS1_2)
     ssl->transform_in = ssl->transform_negotiate;
+#endif
     ssl->session_in = ssl->session_negotiate;
 
+
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
     if( ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM )
     {