Add back SNI related code to validate_certificate

Change-Id: I75883858016d4163cd7c64c3418eb3ca24fa46ea
Signed-off-by: XiaokangQian <xiaokang.qian@arm.com>
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index 239be53..17efa8c 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -560,7 +560,14 @@
      * from the configuration. */
 #if defined(MBEDTLS_SSL_SRV_C)
     if( ssl->conf->endpoint == MBEDTLS_SSL_IS_SERVER )
-        authmode = ssl->conf->authmode;
+    {
+#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
+        if( ssl->handshake->sni_authmode != MBEDTLS_SSL_VERIFY_UNSET )
+            authmode = ssl->handshake->sni_authmode;
+        else
+#endif
+            authmode = ssl->conf->authmode;
+    }
 #endif
 
     /*