[C] Include result slot in getItem for CraftInventoryAnvil. Fixes BUKKIT-2788

AnvilInventory's .getItem method currently throws an IndexOutOfBounds exception when attempting to access Slot #2 (The result slot).  This commit fixes that in the same way its handled by other inventory classes.

LEAKY: https://bukkit.atlassian.net/browse/BUKKIT-2788