Make more auto variables const

That way we know we don't have to worry about saving and restoring their
value.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 221071b..48bb336 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -4313,8 +4313,15 @@
 {
     int ret = MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE;
     size_t i, n;
-    const mbedtls_ssl_ciphersuite_t *ciphersuite_info = ssl->transform_negotiate->ciphersuite_info;
-    int authmode = ssl->conf->authmode;
+    const mbedtls_ssl_ciphersuite_t * const ciphersuite_info =
+          ssl->transform_negotiate->ciphersuite_info;
+#if defined(MBEDTLS_SSL_SRV_C) && defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
+    const int authmode = ssl->handshake->sni_authmode != MBEDTLS_SSL_VERIFY_UNSET
+                       ? ssl->handshake->sni_authmode
+                       : ssl->conf->authmode;
+#else
+    const int authmode = ssl->conf->authmode;
+#endif
     uint8_t alert;
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> parse certificate" ) );
@@ -4338,11 +4345,6 @@
         return( 0 );
     }
 
-#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
-    if( ssl->handshake->sni_authmode != MBEDTLS_SSL_VERIFY_UNSET )
-        authmode = ssl->handshake->sni_authmode;
-#endif
-
     if( ssl->conf->endpoint == MBEDTLS_SSL_IS_SERVER &&
         authmode == MBEDTLS_SSL_VERIFY_NONE )
     {