Add check for unknown nodes.

* Check if node has a definition table before attempting to evaluate its
  attributes.  By application of De Morgan's law, the compound logic
  expression can be rewritten so that the existence of the table is
  checked before all following attribute references.

mods/MAPGEN/mcl_structures/init.lua

@@ -179,7 +179,7 @@ function mcl_structures.generate_igloo(pos, rotation, pr)
 			real_depth = real_depth + 1
 			local node = minetest.get_node({x=tpos.x,y=tpos.y-y,z=tpos.z})
 			local def = minetest.registered_nodes[node.name]
-			if (not def) or (not def.walkable) or (def.liquidtype ~= "none") or (not def.is_ground_content) then
+			if not (def and def.walkable and def.liquidtype == "none" and def.is_ground_content) then
 				bpos.y = tpos.y-y+1
 				break
 			end