Fix PHP Warning: Use of undefined constant IDNA_DEFAULT on systems without php-intl (#6244)

CHANGELOG

@@ -1,6 +1,8 @@
 CHANGELOG Roundcube Webmail
 ===========================
 
+- Fix PHP Warning: Use of undefined constant IDNA_DEFAULT on systems without php-intl (#6244)
+
 RELEASE 1.3.6
 -------------
 - Fix parsing date strings (e.g. from a Date: mail header) with comments (#6216)

program/lib/Roundcube/rcube_utils.php

@@ -905,7 +905,7 @@ class rcube_utils
         // Note that in PHP 7.2/7.3 calling idn_to_* functions with default arguments
         // throws a warning, so we have to set the variant explicitely (#6075)
         $variant = defined('INTL_IDNA_VARIANT_UTS46') ? INTL_IDNA_VARIANT_UTS46 : null;
-        $options = IDNA_DEFAULT;
+        $options = 0;
 
         // Because php-intl extension lowercases domains and return false
         // on invalid input (#6224), we skip conversion when not needed