Additional check for validity of merci_item_nid

I am using a custom form for item reservation which consists of checkboxes instead of select fields. When the form is submitted, deselected items have a merci_item_nid of 0.
In the function merci_add_reservation_items it is not checked whether merci_item_nid is > 0, therefore placeholder content is created which does not have a content type assigned.
The attached patch fixes the problem for me.