Fix bug in moving a larger sized stack in inventory on top of another

- You have 2 stacks: a with x items and b with y<x items
- Take a with your mouse and place it on b
- Whole stack a moves to b
- (y * 2 - x) items move from b to a
- (x - y) items are left to the mouse cursor

src/guiFormSpecMenu.cpp

@@ -1057,7 +1057,7 @@ bool GUIFormSpecMenu::OnEvent(const SEvent& event)
 			if(leftover.count == stack_from.count)
 			{
 				// Swap the stacks
-				m_selected_amount -= stack_to.count;
+				m_selected_amount = stack_to.count;
 			}
 			else if(leftover.empty())
 			{