Fix incorrect assignments content-mathml extension, and don't modify original document. Resolves issue #1575.

Author: dpvc

unpacked/extensions/MathML/content-mathml.js

@@ -1317,9 +1317,11 @@ MathJax.Extension["MathML/content-mathml"] = (function(HUB) {
             } else if (arg.nodeName === 'apply' && children.length === 2 && children[0].nodeName === 'minus') {
               CToP.appendToken(mrow,'mo','\u2212');
               CToP.applyTransform(mrow,children[1],2);
-            } else if (arg.nodeName === 'apply' && children.length>2 && children[0].nodeName === 'times' && children[1].nodeName === 'cn' && ( n = Number(CToP.getTextContent(children[1])) < 0)) {
+            } else if (arg.nodeName === 'apply' && children.length>2 && children[0].nodeName === 'times' && children[1].nodeName === 'cn' && (n = Number(CToP.getTextContent(children[1]))) < 0) {
               CToP.appendToken(mrow,'mo','\u2212');
-              CToP.getTextContent(children[1]) = -n;// fix me: modifying document
+              arg = arg.cloneNode(true);
+              children = CToP.getChildren(arg);
+              children[1].textContent = -n;
               CToP.applyTransform(mrow,arg,2);
             } else{
               CToP.appendToken(mrow,'mo','+');