Fixed Bug in RiftGenerator

Fixed a small bug. DDProperties.WorldRiftGenerationEnabled is supposed
to control whether rifts and gateways generate outside of Limbo. I
assume that rifts and gateways are always supposed to generate in Limbo
given the wording of that description. However, if the flag was set to
false, we would also disable rift generation in Limbo. I've fixed this
by ignoring that flag if we detect that the chunk is in Limbo.