Fix possible hash collision in doors module

[tolmar]

Yes I actually hit a hash collision. Yay me.

Thanks phoenixbound for identifying this actually was a hash collision.

Testing: I just recompiled a project with this change, and went through a few doors.

[charasyn]

Just to be clear, the bug is that we are using the hash collision as the key in the dictionary. This shouldn't be done, since if we have a hash collision then the two keys will compare equal (since the hash is the key).

When we use the block itself as the key, this is avoided since although the two blocks have the same hash, they will not compare equal, so destination_block in destination_locations will be false and we won't incorrectly overwrite the collided-with door entry.