xen-front-pgdir-shbuf: don't record wrong grant handle upon error

In order for subsequent unmapping to not mistakenly unmap handle 0,
record a perceived always-invalid one instead.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://lore.kernel.org/r/82414b0f-1b63-5509-7c1d-5bcc8239a3de@suse.com


Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>