CCL chokes on multiple end-worlds

[yuuka-miya]

First reported in ForgeEssentials/ForgeEssentialsMain#1597.

Affects 1.7.10 and potentially 1.8.

Perhaps a null-check should be added here? https://github.com/Chicken-Bones/CodeChickenLib/blob/master/src/codechicken/lib/world/WorldExtension.java#L53