Guard accesses to Crop::crop_ratios (#5701)

rtgui/crop.h

@@ -18,7 +18,7 @@
  */
 #pragma once
 
-#include <vector>
+#include <memory>
 
 #include <gtkmm.h>
 
@@ -91,16 +91,13 @@ public:
     void rotateCrop         (int deg, bool hflip, bool vflip);
 
 private:
-    struct CropRatio {
-        Glib::ustring label;
-        double value;
-    };
-
-    std::vector<CropRatio> crop_ratios;
+    class CropRatios;
 
     void adjustCropToRatio();
     void updateCurrentRatio();
 
+    const std::unique_ptr<CropRatios> crop_ratios;
+
     Gtk::CheckButton* fixr;
     MyComboBoxText* ratio;
     MyComboBoxText* orientation;