[fix] Multilanguage: Checking proper default menu item in mod_menu

Same issue as in https://github.com/joomla/joomla-cms/pull/4994 which was merged and solved the pathway if in the _menu table the row mainmenu with Home set to Language All, i.e. *, is deleted.

See specifically the comment https://github.com/joomla/joomla-cms/pull/4994#issuecomment-61855825
To test use a multilang site, delete the said menu item.
Create a tag and apply to some articles. Display these articles once at least in frontend.
Create a Popular Tags module and display in frontend.
As $menu->getDefault() does not exist, you will get many notices including
Notice: Trying to get property of non-object in ROOT\modules\mod_menu\helper.php on line 40

Patch and retest