Don't try to typset mathjax if Hub has not loaded.

In our tests there are times when Mathjax has been initialized but
Hub is still undefied.  In this case don't queue up typesetting.

common/static/common/js/discussion/utils.js

@@ -485,7 +485,7 @@
         };
 
         DiscussionUtil.typesetMathJax = function(element) {
-            if (typeof MathJax !== 'undefined' && MathJax !== null) {
+            if (typeof MathJax !== 'undefined' && MathJax !== null && MathJax.Hub !== 'undefined') {
                 MathJax.Hub.Queue(['Typeset', MathJax.Hub, element[0]]);
             }
         };