Bug 765372 - Add some null safety to isElementClickable. r=wesj

mobile/android/chrome/content/browser.js

@@ -3342,7 +3342,7 @@ const ElementTouchHelper = {
     if (!aAllowBodyListeners && aElement && aElement.ownerDocument)
       stopNode = aElement.ownerDocument.body;
 
-    for (let elem = aElement; elem != stopNode; elem = elem.parentNode) {
+    for (let elem = aElement; elem && elem != stopNode; elem = elem.parentNode) {
       if (aUnclickableCache && aUnclickableCache.indexOf(elem) != -1)
         continue;
       if (this._hasMouseListener(elem))