Fix use of uninitialized cursor variable

[bearzly]

The cursor_ variable was being default initialized, which causes undefined behaviour when accessing properties in
ScreenInteractive::Draw. This caused a crash when running with UBSAN.

ftxui/src/ftxui/component/screen_interactive.cpp:852:17: runtime error:
load of value 4195502944, which is not a valid value for type 'Shape'

This change causes cursor_ to be value initialized so that its members will be initialized to 0.

[ArthurSonzogni]

Thanks! This is greatly appreciated. Maybe I should run the fuzzers with UBSAN too instead of only ASAN.

Could we instead initialize Screen::Cursor::shape_ where is is declared?

class Screen {
  ...
  struct Cursor {
    int x = 0;
    int y = 0;

    enum Shape {
      Hidden = 0,
      BlockBlinking = 1,
      Block = 2,
      UnderlineBlinking = 3,
      Underline = 4,
      BarBlinking = 5,
      Bar = 6,
    };
-   Shape shape;
+   Shape shape = Hidden;
  };
  ...
}

[bearzly]

Done, tested that this also fixes the issue.

[ArthurSonzogni]

Thanks! LGTM!