Zero initialize all pointer class and struct members

This prevents the pitfall of UB when checking if they have been
assigned something valid by comparing to nullptr.

scene/gui/menu_button.h

@@ -40,7 +40,7 @@ class MenuButton : public Button {
 	bool clicked = false;
 	bool switch_on_hover = false;
 	bool disable_shortcuts = false;
-	PopupMenu *popup;
+	PopupMenu *popup = nullptr;
 
 	Vector2i mouse_pos_adjusted;