Don't grow papyrus into trees

mods/default/mapgen.lua

@@ -69,14 +69,26 @@ end
 function default.make_papyrus(pos, size)
 	for y=0,size-1 do
 		local p = {x=pos.x, y=pos.y+y, z=pos.z}
-		minetest.env:set_node(p, {name="default:papyrus"})
+		local nn = minetest.env:get_node(p).name
+		if minetest.registered_nodes[nn] and
+			minetest.registered_nodes[nn].buildable_to then
+			minetest.env:set_node(p, {name="default:papyrus"})
+		else
+			return
+		end
 	end
 end
 
 function default.make_cactus(pos, size)
 	for y=0,size-1 do
 		local p = {x=pos.x, y=pos.y+y, z=pos.z}
-		minetest.env:set_node(p, {name="default:cactus"})
+		local nn = minetest.env:get_node(p).name
+		if minetest.registered_nodes[nn] and
+			minetest.registered_nodes[nn].buildable_to then
+			minetest.env:set_node(p, {name="default:cactus"})
+		else
+			return
+		end
 	end
 end