Purchase invoice created for one supplier contains lines from another suppliers order

[steveblamey]

To reproduce:

1. Create invoice from an order and select some lines. Don't save.
2. Go to another order for a different supplier, create invoice and select some lines. Save.
3. The order for the second supplier now has all the lines selected from both suppliers.

Cancelling or deleting lines will have now impact on the order at this point. Invoiced lines will remain invoiced.