Fix NPEs with mod container inventory views with no Bukkit wrapper. Fixes #964.

Container getBukkitView() returns null for modded inventories (not overridden
in subclass), without a Bukkit wrapper for InventoryView. This null inventory
view is sent to plugins via inventory events, which reasonably expect a non-
null inventory, causing event handling failure spam - especially with
InventoryCloseEvents. To solve this, for now, we skip mod inventories.