有限库存 =负数（库存超卖后改为不能超卖）

https://juan.sltestteam.shop/products/%E4%B8%80%E4%BA%8C%E4%B8%89%E5%9B%9B%E4%BA%94%E5%85%AD%E4%B8%83%E5%85%AB%E4%B9%9D%E5%8D%81%E4%B8%80%E4%BA%8C%E4%B8%89%E5%9B%9B%E4%BA%94%E5%85%AD%E4%B8%83%E5%85%AB%E4%B9%9D%E5%8D%81%E4%B8%80%E4%BA%8C%E4%B8%89%E5%9B%9B%E4%BA%94%E5%85%AD%E4%B8%83%E5%85%AB%E4%B9%9D%E5%8D%81-%E6%88%91%E6%98%AF%E5%81%87%E7%9A%84-%E4%B8%80%E4%BA%8C%E4%B8%89%E5%9B%9B%E4%B8%80%E4%BA%8C%E4%B8%89%E5%9B%9B