Fix crash when client disconnects with modded inventory open. Fixes #942.

Client disconnects now send onClose() in recent CraftBukkits; however
onClose() from the IInventory interface is not implemented by mod
inventories so we need to handle this case gracefully. Fixes server crash
java.lang.AbstractMethodError at net.minecraft.inventory.Container.transferTo(Container.java:45)
when a client disconnects while a mod inventory window is open.