IE8: fix namespace selection in link wizard FS#2391

IE8 can't substr() with negative offsets.

lib/scripts/linkwiz.js

@@ -186,7 +186,7 @@ var dw_linkwiz = {
      */
     resultClick: function(a){
         dw_linkwiz.$entry.val(a.title);
-        if(a.title == '' || a.title.substr(-1) == ':'){
+        if(a.title == '' || a.title.substr(a.title.length-1) == ':'){
             dw_linkwiz.autocomplete_exec();
         }else{
             if (jQuery(a.nextSibling).is('span')) {