Test stock_date_end in context before comparing with today

(transplanted from 25baf0c9daa6d35033b21ff41311384b43179833)

product.py

@@ -40,6 +40,7 @@ class Product(OSV):
             return dict([(id, 0.0) for id in ids])
 
         if name == 'quantity' and \
+                context.get('stock_date_end') and \
                 context.get('stock_date_end') > \
                 date_obj.today(cursor, user, context=context):