fix UB in story_viewer

previously the first image timer would not be cleared when the user restarted
the part, which in particular caused UB if the story_viewer was closed
before all timers have finished.

fixes #3838 fixes #3837

src/gui/dialogs/story_viewer.cpp

@@ -67,14 +67,19 @@ story_viewer::story_viewer(const std::string& scenario_name, const config& cfg_p
 	update_current_part_ptr();
 }
 
-story_viewer::~story_viewer()
+void story_viewer::clear_image_timer()
 {
 	if(timer_id_ != 0) {
 		remove_timer(timer_id_);
 		timer_id_ = 0;
 	}
 }
 
+story_viewer::~story_viewer()
+{
+	clear_image_timer();
+}
+
 void story_viewer::pre_show(window& window)
 {
 	window.set_enter_disabled(true);
@@ -294,7 +299,8 @@ void story_viewer::display_part(window& window)
 	text_label.set_label(part_text);
 
 	begin_fade_draw(true);
-
+	// if the previous page was skipped, it is possible that we already have a timer running.
+	clear_image_timer();
 	//
 	// Floating images (handle this last)
 	//

src/gui/dialogs/story_viewer.hpp

@@ -52,6 +52,8 @@ class story_viewer : public modal_dialog
 	/** Inherited from modal_dialog. */
 	virtual void pre_show(window& window) override;
 
+	void clear_image_timer();
+
 	void update_current_part_ptr();
 
 	void display_part(window& window);