Fix flammable item entities crashing

* backported from https://github.com/minetest/minetest_game/pull/2659
* fix to close https://codeberg.org/minenux/minetest-game-minetest/issues/11

mods/default/item_entity.lua

@@ -15,8 +15,8 @@ local item = {
 
 	burn_up = function(self)
 		-- disappear in a smoke puff
-		self.object:remove()
 		local p = self.object:get_pos()
+		self.object:remove()
 		minetest.sound_play("default_item_smoke", {
 			pos = p,
 			max_hear_distance = 8,
@@ -48,6 +48,10 @@ local item = {
 			if self.ignite_timer > 10 then
 				self.ignite_timer = 0
 
+				local pos = self.object:get_pos()
+				if pos == nil then
+					return -- object already deleted
+				end
 				local node = minetest.get_node_or_nil(self.object:get_pos())
 				if not node then
 					return