Address some comments

Hostname_len has at least one byte
Change structure serialized_session_tls13
Fix various issues

Signed-off-by: Xiaokang Qian <xiaokang.qian@arm.com>
diff --git a/library/ssl_client.c b/library/ssl_client.c
index bd9edf1..2c5f664 100644
--- a/library/ssl_client.c
+++ b/library/ssl_client.c
@@ -79,7 +79,7 @@
     {
         ssl->session_negotiate->hostname = mbedtls_calloc( 1, hostname_len );
         if( ssl->session_negotiate->hostname == NULL )
-            return MBEDTLS_ERR_SSL_ALLOC_FAILED;
+            return( MBEDTLS_ERR_SSL_ALLOC_FAILED );
         memcpy(ssl->session_negotiate->hostname, ssl->hostname, hostname_len);
     }
     ssl->session_negotiate->hostname_len = hostname_len;