Fire: omit sounds if sound API not supported by lame fire mods :)

tenplus1's version of the fire mod removes all sound support. This
code makes this mod work if people are using his version of the
fire mod.

init.lua

@@ -200,7 +200,9 @@ minetest.register_node("lightning:dying_flame", {
 
 	on_construct = function(pos)
 		minetest.get_node_timer(pos):start(rng:next(20, 40))
+		if fire and fire.on_flame_add_at then
 			minetest.after(0.5, fire.on_flame_add_at, pos)
+		end
 	end,
 })