MAGETWO-96941: TinyMCE editor JSON attribute improvement

- Added additional JSON check to TinyMCE

Author: Hwashiang Yu

lib/web/tiny_mce/plugins/media/editor_plugin_src.js

@@ -372,9 +372,15 @@
 			};
 
 			data = node.attr('data-mce-json');
-			if (!data)
+			if (!data || !data.isJSON())
 				return;
 
+			try {
+				data.evalJSON(true);
+			} catch {
+				return;
+			}
+
 			data = JSON.parse(data);
 			typeItem = this.getType(node.attr('class'));