luci-base: cbi.js: handle undefined arguments in formatting

Fix the JavaScript String.format() to not trigger an exception if the argument
for an escaped format like %h or %q is undefined.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/htdocs/luci-static/resources/cbi.js

@@ -1300,6 +1300,9 @@ String.prototype.format = function()
 	var quot_esc = [/"/g, '"', /'/g, '''];
 
 	function esc(s, r) {
+		if (typeof(s) !== 'string' && !(s instanceof String))
+			return '';
+
 		for( var i = 0; i < r.length; i += 2 )
 			s = s.replace(r[i], r[i+1]);
 		return s;