fix crash occurs when listbox is selecting item after pressing shift(#312)

source/gui/widgets/listbox.cpp

@@ -1522,9 +1522,37 @@ namespace nana
 					return n;
 				}
 
-				template<typename Pred>
+				/// Finds a good item or category if an item specified by pos is invaild
-				std::vector<std::pair<index_pair, bool>> select_display_range_if(const index_pair& fr_abs, index_pair to_dpl, bool deselect_others, Pred pred)
+				index_pair find_next_good(index_pair pos, bool ignore_category) const noexcept
 				{
+					//Return the pos if it is good
+					if (this->good(pos))
+						return pos;
+					
+					while(pos.cat < this->categories_.size())
+					{
+						if ((pos.npos == pos.item) && !ignore_category)
+							return pos;
+
+						auto cat_item_size = this->get(pos.cat)->items.size();
+
+						if (pos.item < cat_item_size)
+							return pos;
+
+						if ((pos.npos == pos.item) && (cat_item_size > 0) && ignore_category)
+							return index_pair{ pos.cat, 0 };
+
+						++pos.cat;
+						pos.item = pos.npos;
+					}
+					return index_pair{};
+				}
+
+				template<typename Pred>
+				std::vector<std::pair<index_pair, bool>> select_display_range_if(index_pair fr_abs, index_pair to_dpl, bool deselect_others, Pred pred)
+				{
+					fr_abs = find_next_good(fr_abs, true);
+					
 					if (to_dpl.empty())
 					{
 						if (fr_abs.empty())
@@ -1532,6 +1560,7 @@ namespace nana
 
 						to_dpl = this->last();
 					}
+					
 
 					auto fr_dpl = (fr_abs.is_category() ? fr_abs : this->index_cast(fr_abs, false));	//Converts an absolute position to display position
                     if (fr_dpl > to_dpl)