Fix Clamp crash when entries_ size is zero (#306)

Run ftxui_example_homescreen on Windows, then select compiler tab, crash on origin code.

Co-authored-by: chenpeizhe <peizhe.chen@horizon.ai>

src/ftxui/screen/util.hpp

@@ -0,0 +1,15 @@
+namespace ftxui {
+namespace util {
+
+// Similar to std::clamp, but allow hi to be lower than lo.
+template <class T>
+constexpr const T& clamp(const T& v, const T& lo, const T& hi) {
+  return v < lo ? lo : hi < v ? hi : v;
+}
+
+}  // namespace util
+}  // namespace ftxui
+
+// Copyright 2022 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.