Mirrored totem glyphs block path to rest of level, including downstairs

[apgove]

Reported at https://www.reddit.com/r/brogueforum/comments/1aknhno/ce_1111_impassable_mirrored_totem/ Seed #354556361, depth 6

This was v1.11.1, but I don't think level generation has changed since then.