FIX: a crash when clear an empty listbox

source/gui/widgets/listbox.cpp

@@ -3326,6 +3326,10 @@ namespace nana
 				void trigger::key_press(graph_reference graph, const arg_keyboard& arg)
 				{
 					bool up = false;
+
+                    if (essence_->lister.size_categ()==1 && essence_->lister.size_item(0)==0)
+                       return ; 
+
 					switch(arg.key)
 					{
 					case keyboard::os_arrow_up: