[fix] do not use category select js if there is no search query field on the page

searx/static/js/search_on_category_select.js

 $(document).ready(function() {
-    if($('#q')) {
+    if($('#q').length) {
         $('#categories label').click(function(e) {
             $('#categories input[type="checkbox"]').each(function(i, checkbox) {
                 $(checkbox).prop('checked', false);