fix: app crashes if SELECT results in null (#298)

bookshop/srv/cat-service.js

@@ -8,7 +8,9 @@ class CatalogService extends cds.ApplicationService { init(){
   this.on ('submitOrder', async req => {
     const {book,quantity} = req.data
     if (quantity < 1) return req.reject (400,`quantity has to be 1 or more`)
-    let {stock} = await SELECT `stock` .from (Books,book)
+    let b = await SELECT `stock` .from (Books,book)
+    if (!b) return req.error (404,`Book #${book} doesn't exist`)
+    let {stock} = b
     if (quantity > stock) return req.reject (409,`${quantity} exceeds stock for book #${book}`)
     await UPDATE (Books,book) .with ({ stock: stock -= quantity })
     await this.emit ('OrderedBook', { book, quantity, buyer:req.user.id })