js/ui: check whether a link tag has a type

The new favicon didn't have such, resulting in JS error when reloading
CSS at runtime.

public/js/icinga/ui.js

@@ -100,7 +100,7 @@
             icinga.logger.info('Reloading CSS');
             $('link').each(function() {
                 var $oldLink = $(this);
-                if ($oldLink.attr('type').indexOf('css') > -1) {
+                if ($oldLink.hasAttr('type') && $oldLink.attr('type').indexOf('css') > -1) {
                     var $newLink = $oldLink.clone().attr(
                         'href',
                         icinga.utils.addUrlParams(