reception: fix bug putting a product to edition and validating without change

reception/static/js/reception_produits.js

@@ -1349,7 +1349,8 @@ function editProductInfo (productToEdit, value = null, batch = false) {
             firstUpdate = (index == -1); //first update
 
             if (productToEdit.product_qty != newValue) {
-                if (firstUpdate) {
+                // If no old_qty in productToEdit, product qty wasn't edited before
+                if (productToEdit.old_qty === undefined) {
                     productToEdit.old_qty = productToEdit.product_qty;
                 } else {
                     //if it is not the first update AND newValue is equal to the validation qty then the product is valid (qty not changed)