Nether nodes can't be destroyed by Dungeon Masters

- Adding a check for the nodes hit by a dungeon master's fireball. If the
  node is under y=-19600, is not nether:rack, and is part of the nether
  mod, it won't be destroyed.

mods/mobs/dungeonmaster.lua

@@ -92,6 +92,9 @@ mobs:register_arrow("mobs:fireball", {
 				for dz=-1,1 do
 					local p = {x=pos.x+dx, y=pos.y+dy, z=pos.z+dz}
 					local n = minetest.get_node(p).name
+					if p.y < -19600 and n ~= "nether:rack" and n:split(":")[1] == "nether" then
+						return
+					end
 					if n ~= "default:obsidian" and n ~= "ethereal:obsidian_brick" then	
 					if minetest.registered_nodes[n].groups.flammable or math.random(1, 100) <= 30 then
 						minetest.set_node(p, {name="fire:basic_flame"})