Fix an assumption in the ad check to handle AdBlock for Chrome.

This fixes the bug where the submit page initializes unsuccessfully,
causing link posts to be accidentally submitted as text posts.

r2/r2/templates/ads.html

@@ -37,7 +37,7 @@
      %>
     $(function() {
       var ad = $("#ad-frame");
-      if(ad.height() == 0 || ad.width() == 0 || ad.offset().left == 0) {
+      if(!ad.length || ad.height() == 0 || ad.width() == 0 || ad.offset().left == 0) {
         $(".footer").append("<img alt='' src='${unsafe(tracker_url)}&random=" +
                                     Math.random()*10000000000000000 + "'/>");
       }