fix(thread): 在线程外调用api.id时添加错误检查

添加对current变量的检查，防止在线程外调用api.id时导致空指针异常

data/computercraft/lua/rom/modules/main/rc/thread.lua

@@ -103,6 +103,9 @@ function api.exists(id)
 end
 
 function api.id()
+  if not current then
+    error("No current thread - are you calling this outside a thread?")
+  end
   return current.id
 end