Fix entity teleportation when dismounting. Fixes BUKKIT-4591

When an entity dismounts another entity it is teleported to a location
near the entity that it just dismounted from. This location is dependent
upon the vehicle's boundingBox.b vale, which, for some entities, can have
a value of NaN. The entity is then teleported to a location with a y value
of NaN, which is an invalid location for entities.

This commit fixes the issue by using the vehicle's y location in place of
boundingBox.b if boundingBox.b is NaN.