[FIX] mrp: crash onchange

In a MO, change a product in "Consumed Materials" by clicking on "Search
more...". It crashes.

Clicking on "Search more..." triggers the onchange since it empties the
line. Since there is no product on the line anymore, it crashes.

opw-705644

addons/mrp/models/stock_move.py

@@ -87,7 +87,7 @@ class StockMove(models.Model):
             # For consumables, state is available so availability = qty to do
             if move.state == 'assigned':
                 move.quantity_available = move.product_uom_qty
-            else:
+            elif move.product_id.uom_id and move.product_uom:
                 move.quantity_available = move.product_id.uom_id._compute_quantity(move.reserved_availability, move.product_uom)
 
     @api.multi