Workaround for concrete powder hardening ABM crash

mods/ITEMS/mcl_colorblocks/init.lua

@@ -169,7 +169,12 @@ minetest.register_abm({
 	neighbors = {"group:water"},
 	action = function(pos, node)
 		local harden_to = minetest.registered_nodes[node.name]._mcl_colorblocks_harden_to
-		minetest.swap_node(pos, { name = harden_to, param = node.param, param2 = node.param2 })
+		-- It should be impossible for harden_to to be nil, but a Minetest bug might call
+		-- the ABM on the new concrete node, which isn't part of this ABM!
+		if harden_to then
+			node.name = harden_to
+			minetest.set_node(pos, node)
+		end
 	end,
 })