Correct an AIOOBE when dropping items in Creative mode. Fixes BUKKIT-4305

When a Player drops an ItemStack while in creative mode by placing it
outside of their inventory window, the slot number in the packet is -1.
The check that was added to avoid throwing InventoryCreativeEvent
excessively didn't take this into account, and would cause an
ArrayIndexOutOfBoundsException to be thrown when attempting to get the
ItemStack currently in the slot specified by the packet.

This change will avoid the check if slot number was less than 0, which
indicates that the item is being dropped.