Mirror imp replaces stairs

[red-kangaroo]

If a mirror imp croaks exactly on the stairs it replaces the tile with the crystal they leave and an engraving describing the staircase.

[red-kangaroo]

The easiest fix is this:

void mirrorimp::CreateCorpse (lsquare *Square) {
  // do not replace any interesting terrain
  if (!Square->GetOLTerrain()) {
    decoration *Shard = decoration::Spawn(SHARD);
    Shard->InitMaterials(MAKE_MATERIAL(GLASS));
    Square->ChangeOLTerrainAndUpdateLights(Shard);
  }
  SendToHell();
}