Igloo structure can spawn above ground

The bug

The igloo structure can spawn above the ground.

How to reproduce

1. Create a customized world with the seed 48572 and the following preset:

   {"coordinateScale":684.412,"heightScale":684.412,"lowerLimitScale":512.0,"upperLimitScale":512.0,"depthNoiseScaleX":200.0,"depthNoiseScaleZ":200.0,"depthNoiseScaleExponent":0.5,"mainNoiseScaleX":80.0,"mainNoiseScaleY":160.0,"mainNoiseScaleZ":80.0,"baseSize":8.5,"stretchY":12.0,"biomeDepthWeight":1.0,"biomeDepthOffset":0.0,"biomeScaleWeight":1.0,"biomeScaleOffset":0.0,"seaLevel":63,"useCaves":true,"useDungeons":true,"dungeonChance":100,"useStrongholds":true,"useVillages":true,"useMineShafts":true,"useTemples":true,"useMonuments":true,"useRavines":true,"useWaterLakes":true,"waterLakeChance":40,"useLavaLakes":true,"lavaLakeChance":80,"useLavaOceans":false,"fixedBiome":-1,"biomeSize":1,"riverSize":4,"dirtSize":33,"dirtCount":40,"dirtMinHeight":0,"dirtMaxHeight":256,"gravelSize":33,"gravelCount":40,"gravelMinHeight":0,"gravelMaxHeight":256,"graniteSize":33,"graniteCount":40,"graniteMinHeight":0,"graniteMaxHeight":80,"dioriteSize":33,"dioriteCount":40,"dioriteMinHeight":0,"dioriteMaxHeight":80,"andesiteSize":33,"andesiteCount":40,"andesiteMinHeight":0,"andesiteMaxHeight":80,"coalSize":17,"coalCount":40,"coalMinHeight":0,"coalMaxHeight":128,"ironSize":9,"ironCount":40,"ironMinHeight":0,"ironMaxHeight":64,"goldSize":9,"goldCount":40,"goldMinHeight":0,"goldMaxHeight":32,"redstoneSize":8,"redstoneCount":40,"redstoneMinHeight":0,"redstoneMaxHeight":16,"diamondSize":8,"diamondCount":40,"diamondMinHeight":0,"diamondMaxHeight":16,"lapisSize":7,"lapisCount":40,"lapisCenterHeight":16,"lapisSpread":16} 
   

2. /tp @p -921 80 294