Refactor server hello

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index f31e7ab..2883328 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -2001,6 +2001,7 @@
     size_t output_len;
 
     *out_len = 0;
+    ssl->handshake->sent_extensions = MBEDTLS_SSL_EXT_MASK_NONE;
 
     /* ...
      * ProtocolVersion legacy_version = 0x0303; // TLS 1.2