Address potential crash if a storyscreen has no showable parts

src/gui/dialogs/story_viewer.hpp

@@ -37,7 +37,12 @@ public:
 
 	static void display(const std::string& scenario_name, const config& story, CVideo& video)
 	{
-		story_viewer(scenario_name, story).show(video);
+		if(!story.has_child("part")) {
+			return;
+		}
+		try {
+			story_viewer(scenario_name, story).show(video);
+		} catch(std::out_of_range&) {}
 	}
 
 private:

src/storyscreen/controller.hpp

@@ -39,7 +39,7 @@ public:
 
 	part_pointer_type get_part(int index) const
 	{
-		return parts_[index];
+		return parts_.at(index);
 	}
 
 	int max_parts() const