Fix nodes that should not burn up burning up

Nodes with the group “flammable = -1” (e.g. crafting tables) must be
able to catch fire, but must not burn up. This patch adds checks for
this group.

mods/ITEMS/mcl_fire/init.lua

@@ -27,6 +27,10 @@ local spawn_smoke = function(pos)
 	}, "high")
 end
 
+local function has_flammable(pos)
+	return minetest.find_node_near(pos, 1, {"group:flammable"})
+end
+
 --
 -- Items
 --
@@ -104,7 +108,8 @@ minetest.register_node("mcl_fire:fire", {
 	drop = "",
 	sounds = {},
 	on_timer= function(pos)
-		if not minetest.find_node_near(pos, 1, {"group:flammable"}) then
+		local p=has_flammable(pos)
+		if not p or minetest.get_item_group(minetest.get_node(p).name, "flammable") == -1 then
 			minetest.remove_node(pos)
 			return
 		end
@@ -339,10 +344,6 @@ minetest.register_abm({
 -- [...]a fire that is not adjacent to any flammable block does not spread, even to another flammable block within the normal range.
 -- https://minecraft.fandom.com/wiki/Fire#Spread
 
-local function has_flammable(pos)
-	return minetest.find_node_near(pos, 1, {"group:flammable"})
-end
-
 local function check_aircube(p1,p2)
 	local nds=minetest.find_nodes_in_area(p1,p2,{"air"})
 	for k,v in pairs(nds) do
@@ -415,15 +416,18 @@ else -- Fire enabled
 		chance = 18,
 		catch_up = false,
 		action = function(pos)
-			local p = minetest.find_node_near(pos, 1, {"group:flammable"})
+			local p = has_flammable(pos)
 			if not p then
 				return
 			end
-			local flammable_node = minetest.get_node(p)
-			local def = minetest.registered_nodes[flammable_node.name]
-			if def.on_burn then
-				def.on_burn(p)
-			else
+
+			local nn = minetest.get_node(p).name
+			local def = minetest.registered_nodes[nn]
+			local fgroup = minetest.get_item_group(nn, "flammable")
+
+			if def and def._on_burn then
+				def._on_burn(p)
+			elseif fgroup ~= -1 then
 				minetest.swap_node(p, {name = "mcl_fire:fire"})
 				fire_timer(p)
 				minetest.check_for_falling(p)