[5.6] mark places with unsafe set_hp entity handling

There might be other places i might be missing, but these are the obvious ones.

leftover entity handling i did not mark:
* everything that involves a apply()-like algorithm (e.g. callbacks) over the
  same list of objects.
* in mcl_damage, it's not known whether mcl_damage.from_mt() would involve
  entity handling or just player handling.

mods/CORE/mcl_explosions/init.lua

@@ -337,12 +337,13 @@ local function trace_explode(pos, strength, raydirs, radius, info, direct, sourc
 						if not obj:is_player() then
 							return
 						end
-
+						-- ERROR: [#2523] THIS WILL CAUSE INVALID OBJECTREFS
 						mcl_util.deal_damage(obj, damage, {type = "explosion", direct = direct, source = source})
 
 						obj:add_velocity(vector.multiply(punch_dir, impact * 20))
 					end)
 				else
+					-- ERROR: [#2523] THIS WILL CAUSE INVALID OBJECTREFS
 					mcl_util.deal_damage(obj, damage, {type = "explosion", direct = direct, source = source})
 
 					if obj:is_player() or ent.tnt_knockback then

mods/ENTITIES/mcl_boats/init.lua

@@ -201,6 +201,7 @@ function boat.on_punch(self, puncher, time_from_last_punch, tool_capabilities, d
 end
 
 function boat.on_step(self, dtime, moveresult)
+	-- ERROR: [#2523] THIS WILL CAUSE INVALID OBJECTREFS
 	mcl_burning.tick(self.object, dtime, self)
 
 	self._v = get_v(self.object:get_velocity()) * get_sign(self._v)

mods/ENTITIES/mcl_mobs/api.lua

@@ -3617,6 +3617,7 @@ local mob_step = function(self, dtime)
 	check_item_pickup(self)
 	check_aggro(self,dtime)
 	if not self.fire_resistant then
+		-- ERROR: [#2523] THIS WILL CAUSE INVALID OBJECTREFS
 		mcl_burning.tick(self.object, dtime, self)
 	end
 

mods/ENVIRONMENT/lightning/init.lua

@@ -120,6 +120,8 @@ function lightning.strike(pos)
 	if not pos then
 		return false
 	end
+	-- ERROR: [#2523] THIS WILL CAUSE INVALID OBJECTREFS
+	-- mcl_util.deal_damage inside one of on_strike_functions
 	local objects = get_objects_inside_radius(pos2, 3.5)
 	if lightning.on_strike_functions then
 		for _, func in pairs(lightning.on_strike_functions) do
@@ -174,6 +176,7 @@ lightning.register_on_strike(function(pos, pos2, objects)
 		elseif lua and lua.name == "mobs_mc:creeper" then
 			mcl_util.replace_mob(obj, "mobs_mc:creeper_charged")
 		else
+			-- ERROR: [#2523] THIS WILL CAUSE INVALID OBJECTREFS
 			mcl_util.deal_damage(obj, 5, { type = "lightning_bolt" })
 		end
 	end

mods/ITEMS/mcl_armor/damage.lua

@@ -89,6 +89,7 @@ mcl_damage.register_modifier(function(obj, damage, reason)
 	local thorns_damage = thorns_damage_regular + thorns_damage_irregular
 
 	if thorns_damage > 0 and reason.type ~= "thorns" and reason.source ~= obj then
+		-- ERROR: [#2523] THIS WILL CAUSE INVALID OBJECTREFS
 		mcl_util.deal_damage(reason.source, thorns_damage, {type = "thorns", direct = obj})
 
 		local thorns_item = thorns_pieces[math.random(#thorns_pieces)]

mods/ITEMS/mcl_bows/arrow.lua

@@ -114,6 +114,7 @@ local function damage_particles(pos, is_critical)
 end
 
 function ARROW_ENTITY.on_step(self, dtime)
+	-- ERROR: [#2523] THIS WILL CAUSE INVALID OBJECTREFS
 	mcl_burning.tick(self.object, dtime, self)
 
 	self._time_in_air = self._time_in_air + .001

mods/ITEMS/mcl_bows/rocket.lua

@@ -312,6 +312,7 @@ local function damage_particles(pos, is_critical)
 end
 
 function ARROW_ENTITY.on_step(self, dtime)
+	-- ERROR: [#2523] THIS WILL CAUSE INVALID OBJECTREFS
 	mcl_burning.tick(self.object, dtime, self)
 
 	self._time_in_air = self._time_in_air + .001