Fix: If a footer is found to be null or undefined, Excel export could…

… throw an error

https://datatables.net/forums/discussion/78306

js/buttons.html5.js

@@ -516,7 +516,7 @@ function _excelColWidth(data, col) {
 	var max = data.header[col].length;
 	var len, lineSplit, str;
 
-	if (data.footer && data.footer[col].length > max) {
+	if (data.footer && data.footer[col] && data.footer[col].length > max) {
 		max = data.footer[col].length;
 	}