Parse markdown correctly when mathjax is disabled

IPython/html/static/notebook/js/mathjaxutils.js

@@ -131,7 +131,7 IPython.mathjaxutils = (function (IPython) {
     //
     var remove_math = function (text) {
         if (!window.MathJax) {
-            return text;
+            return [text, null];
         }
 
         var math = []; // stores math strings for later

IPython/html/static/notebook/js/textcell.js

@@ -314,10 +314,11 var IPython = (function (IPython) {
     MarkdownCell.prototype.render = function () {
         if (this.rendered === false) {
             var text = this.get_text();
+            var math = null;
             if (text === "") { text = this.placeholder; }
-            var text_math = IPython.mathjaxutils.remove_math(text);
-            var text = text_math[0];
-            var math = text_math[1];
+            var text_and_math = IPython.mathjaxutils.remove_math(text);
+            text = text_and_math[0];
+            math = text_and_math[1];
             var html = marked.parser(marked.lexer(text));
             html = $(IPython.mathjaxutils.replace_math(html, math));
             // links in markdown cells should open in new tabs
@@ -517,13 +518,14 var IPython = (function (IPython) {
     HeadingCell.prototype.render = function () {
         if (this.rendered === false) {
             var text = this.get_text();
+            var math = null;
             // Markdown headings must be a single line
             text = text.replace(/\n/g, ' ');
             if (text === "") { text = this.placeholder; }
             text = Array(this.level + 1).join("#") + " " + text;
             var text_and_math = IPython.mathjaxutils.remove_math(text);
-            var text = text_and_math[0];
-            var math = text_and_math[1];
+            text = text_and_math[0];
+            math = text_and_math[1];
             var html = marked.parser(marked.lexer(text));
             var h = $(IPython.mathjaxutils.replace_math(html, math));
             // add id and linkback anchor