[3.3.0] Another mimic bug

[flechette]

When a chest mimic that is undiscovered has an item placed under it that is auto-squelched, the symbol on the map goes from the normal ~ to the normal floor tile. This is a dead giveaway that the chest isn't really a chest. This happened with a Large wooden chest (empty) {squelch} mimic. He ... also should have been squelched anyway. Being dlvl 99, should (empty) chest mimics even exist? Can chests even BE empty on generation at that depth?