fix crash on missing props to string_template() (#1523)

Patched up `make_node()` without `orig`.

There may be other cases where `start` could be missing, so make it print "undefined" instead of crashing.

fixes #1518

lib/utils.js

@@ -185,7 +185,7 @@ function push_uniq(array, el) {
 
 function string_template(text, props) {
     return text.replace(/\{(.+?)\}/g, function(str, p){
-        return props[p];
+        return props && props[p];
     });
 };