fix child count of Opt

src/main/jastadd/JastAddAPIExtension.jadd

@@ -26,7 +26,7 @@ aspect JastAddAPIExtension {
       if (child instanceof JastAddList) {
         result += child.numChildren();
       } else if (child instanceof Opt) {
-        if (getChild(0).numChildren() != 0) {
+        if (child.numChildren() != 0) {
           result += 1;
         }
       } else {