Teleports can lead into a locked vault

[tehtmi]

A teleport's destination is chosen right after it is generated. However, it is possible for the layout of the level to change after the teleport is generated. Generally, these changes come from placing other rooms.

This means teleports can have several unfortunate destinations: lava rooms, locked vaults, or occasionally walls. Adding more room types will only aggravate the problem.

One solution would be to generate a teleport's destination the first time it is used, as the level will have finished generating by then.

[epyon]

(In [1657]) * teleport destination is determined at first use, no in-vault teleporting anymore (fixes #221)