Iterate over a copy of keys in products_by_location

As the quantities dictionary may change during the iteration, we must loop over
a copy of the keys.

issue8978
review260771002

product.py

@@ -203,7 +203,7 @@ class Product(StockMixin, object, metaclass=PoolMeta):
 
         if wh_to_add:
             for wh, storage in wh_to_add.items():
-                for key in quantities:
+                for key in list(quantities.keys()):
                     if key[0] == storage:
                         quantities[(wh,) + key[1:]] = quantities[key]
                         if storage in storage_to_remove: