fix(shop): 修复优惠券领取功能的空指针异常问题

- 增加了对优惠券不存在的检查
-安全地检查和更新优惠券的已领取数量，避免空指针异常
- 在 SQL 查询中使用 COALESCE 函数安全地获取已领取数量

src/main/java/com/gxwebsoft/shop/controller/ShopUserCouponController.java

@@ -120,7 +120,15 @@ public class ShopUserCouponController extends BaseController {
       final User loginUser = getLoginUser();
       if (loginUser == null) return fail("请先登录");
       ShopCoupon coupon = couponService.getByIdRel(userCoupon.getCouponId());
-        if (coupon.getTotalCount() != -1 && coupon.getReceiveNum() >= coupon.getTotalCount()) return fail("已经被领完了");
+
+      // 检查优惠券是否存在
+      if (coupon == null) return fail("优惠券不存在");
+
+      // 安全地检查已领取数量，避免空指针异常
+      Integer receiveNum = coupon.getReceiveNum();
+      if (receiveNum == null) receiveNum = 0;
+
+      if (coupon.getTotalCount() != -1 && receiveNum >= coupon.getTotalCount()) return fail("已经被领完了");
         List<ShopUserCoupon> userCouponList = shopUserCouponService.list(
                 new LambdaQueryWrapper<ShopUserCoupon>()
                         .eq(ShopUserCoupon::getCouponId, userCoupon.getCouponId())
@@ -150,7 +158,11 @@ public class ShopUserCouponController extends BaseController {
         }
         userCoupon.setUserId(getLoginUserId());
         shopUserCouponService.save(userCoupon);
-        coupon.setReceiveNum(coupon.getReceiveNum() + 1);
+
+        // 安全地更新已领取数量，避免空指针异常
+        Integer currentReceiveNum = coupon.getReceiveNum();
+        if (currentReceiveNum == null) currentReceiveNum = 0;
+        coupon.setReceiveNum(currentReceiveNum + 1);
         couponService.updateById(coupon);
         return success("领取成功");
     }

src/main/java/com/gxwebsoft/shop/mapper/xml/ShopCouponMapper.xml

@@ -4,7 +4,8 @@
 
     <!-- 关联查询sql -->
     <sql id="selectSql">
-        SELECT a.*
+        SELECT a.*,
+               COALESCE((SELECT COUNT(*) FROM shop_user_coupon suc WHERE suc.coupon_id = a.id AND suc.deleted = 0), 0) AS receive_num
         FROM shop_coupon a
         <where>
             <if test="param.id != null">