fix bug where listbox::item_proxy::empty behaves incorrectly

include/nana/gui/widgets/listbox.hpp

@@ -824,6 +824,10 @@ namespace nana
 				/// Determines whether the item is displayed on the screen
 				bool displayed() const;
 
+				/// Determines whether the item_proxy refers to invalid item.
+				/**
+				 * @return true if the item_proxy refers to an invalid item, false otherwise.
+				 */
 				bool empty() const noexcept;
 
 				/// Checks/unchecks the item

source/gui/widgets/listbox.cpp

@@ -4762,7 +4762,7 @@ namespace nana
 
 				bool item_proxy::empty() const noexcept
 				{
-					return !ess_;
+					return !(ess_ && ess_->lister.good(pos_));
 				}
 
 				item_proxy & item_proxy::check(bool ck, bool scroll_view)