[C] Stop vehicle enter events from being called in an unloaded Chunk. Fixes BUKKIT-3831

Description: 

Currently the VehicleEnterEvent is sent even when a chunk is unloaded, which can cause a crash if .getLocation().getBlock() is called on the vehicle. This can occur when entities are unloaded in chunks, because as of mc 1.5 this is called when a chunk is loaded containing full vehicles.

Justification:

In 1.5, on chunk load all vehicles are re-entered to make sure they are properly saved. This causes an issue for bukkit plugins, because if they try to access the block at which the vehicle is at, it'll cause a crash due to it being an infinite loop of chunk loading.

PR Breakdown:

This PR stops the VehicleEnterEvent being called in an unloaded chunk, to stop the crash caused by the event.

Testing Results and Materials:

To test this, I downloaded a build of craftbook (build.sk89q.com/job/craftbook/) then put a cow in a minecart, unloaded the chunk and let it load again. No crash occurred.

JIRA Ticket:

BUKKIT-3831 - https://bukkit.atlassian.net/browse/BUKKIT-3831