Issue #877: polyfill bugfix

src/mol-util/polyfill.ts

@@ -27,7 +27,7 @@ if (typeof window !== 'undefined') {
     })();
 }
 
-if (typeof window.HTMLCanvasElement !== 'undefined' && !window.HTMLCanvasElement.prototype.toBlob) {
+if (typeof window !== 'undefined' && typeof window.HTMLCanvasElement !== 'undefined' && !window.HTMLCanvasElement.prototype.toBlob) {
     // http://code.google.com/p/chromium/issues/detail?id=67587#57
     Object.defineProperty(window.HTMLCanvasElement.prototype, 'toBlob', {