(fix) JavaScript error when saving preferences

The error was occuring when the user enabled forwarding but no domains
constraints were defined in the SOGo's configuration.

UI/WebServerResources/UIxPreferences.js

@@ -79,7 +79,7 @@ function savePreferences(sender) {
             }
 
         // We check if we can only to internal/external addresses.
-        var constraints = parseInt(forwardConstraints);
+        var constraints = (typeof forwardConstraints != "undefined") && parseInt(forwardConstraints);
         
         if (constraints > 0) {
             // We first extract the list of 'known domains' to SOGo