sunyu::fix bug about combo-product calculate price

shopping-cart-application-service/src/main/java/cn/freemud/service/impl/mcoffee/calculation/SetMealCalculation.java

@@ -86,7 +86,8 @@ public class SetMealCalculation {
                             comboxmaterialPrice+= materialGoods.getFinalPrice() * cartGoods.getQty();
                         }
                     }
-                    comboxGoods.setAmount(comboxGoods.getAmount()+comboxmaterialPrice);
+                    comboxGoods.setAmount((null == comboxGoods.getAmount() ? 0 :
+                            comboxGoods.getAmount())+comboxmaterialPrice);
                     materialPrice = materialPrice + comboxmaterialPrice;
                 }
             }