Refactor client_hello parser and writer

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_client.c b/library/ssl_client.c
index 1c5b447..ebf0fa7 100644
--- a/library/ssl_client.c
+++ b/library/ssl_client.c
@@ -564,7 +564,7 @@
 
 #if defined(MBEDTLS_SSL_PROTO_TLS1_3)
     /* Keeping track of the included extensions */
-    handshake->sent_extensions = MBEDTLS_SSL_EXT_NONE;
+    handshake->sent_extensions = MBEDTLS_SSL_EXT_MASK_NONE;
 #endif
 
     /* First write extensions, then the total length */