Fix gridbox segfault (#260)

The problem was about

src/ftxui/dom/gridbox.cpp

@@ -20,7 +20,7 @@ class GridBox : public Node {
     for (const auto& line : lines_)
       x_size = std::max(x_size, (int)line.size());
     for (auto& line : lines_) {
-      while (line.size() < (size_t)y_size) {
+      while (line.size() < (size_t)x_size) {
         line.push_back(filler());
       }
     }