Fix crash when skeleton tries to jock when there is no pos, maybe because pos is unloaded.

mods/ENTITIES/mcl_mobs/api.lua

@@ -62,8 +62,11 @@ function mob_class:update_tag() --update nametag and/or the debug box
 end
 
 function mob_class:jock_to(mob, reletive_pos, rot)
+	local pos = self.object:get_pos()
+	if not pos then return end
+
 	self.jockey = mob
-	local jock = minetest.add_entity(self.object:get_pos(), mob)
+	local jock = minetest.add_entity(pos, mob)
 	if not jock then return end
 	jock:get_luaentity().docile_by_day = false
 	jock:get_luaentity().riden_by_jock = true

mods/ENTITIES/mobs_mc/skeleton+stray.lua

@@ -91,7 +91,7 @@ local skeleton = {
 	},
 	on_spawn = function(self)
 		if math.random(100) == 1 then
-			self:jock_to("mobs_mc:spider", vector.new(0,0,0), vector.new(0,0,0))
+			self:jock_to("mobs_mc:spider", vector.zero(), vector.zero())
 		end
 		return true
 	end,