Fix bug with container on Windows.

This fix the bug from:
https://github.com/ArthurSonzogni/FTXUI/pull/11

About:
~~~
Bug: Focus handling not working

in the examples (e.g. checkbox.cpp) I can toggle the individual
checkboxes but I cannot move between items, I tried to understand the
focus implementation but am I unsure which keypresses would move focus
between different components
~~~

src/ftxui/component/container.cpp

@@ -42,7 +42,9 @@ bool Container::OnEvent(Event event) {
 Component* Container::ActiveChild() {
   if (children_.size() == 0)
     return nullptr;
-  return children_[*selector_ % children_.size()];
+
+  int selected = selector_ ? *selector_ : selected_;
+  return children_[selected % children_.size()];
 }
 
 bool Container::VerticalEvent(Event event) {