Refine comments for psk related code

Change-Id: Iff5c176bb902919abc8d4fb78a185aa68704a791
Signed-off-by: XiaokangQian <xiaokang.qian@arm.com>
diff --git a/library/ssl_tls13_client.c b/library/ssl_tls13_client.c
index 37e57e8..5c693bd 100644
--- a/library/ssl_tls13_client.c
+++ b/library/ssl_tls13_client.c
@@ -618,7 +618,7 @@
     *out_len = 0;
 
     /* Skip writing extension if no PSK key exchange mode
-     * is enabled in the config or there is no PSK to offer.
+     * is enabled in the config.
      */
     if( !mbedtls_ssl_conf_tls13_some_psk_enabled( ssl ) )
     {
@@ -733,12 +733,12 @@
         return( 0 );
     }
 
-    /*
-     * Ciphersuite list
-     */
-    ciphersuites = ssl->conf->ciphersuite_list;
     if( psk_type == MBEDTLS_SSL_TLS1_3_PSK_EXTERNAL )
     {
+        /*
+         * Ciphersuite list
+         */
+        ciphersuites = ssl->conf->ciphersuite_list;
         for( int i = 0; ciphersuites[i] != 0; i++ )
         {
             ciphersuite_info = mbedtls_ssl_ciphersuite_from_id(