vfio-ccw: Set pa_nr to 0 if memory allocation fails for pa_iova_pfn

So we don't call try to call vfio_unpin_pages() incorrectly.

Fixes: 0a19e61e ("vfio: ccw: introduce channel program interfaces")
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <33a89467ad6369196ae6edf820cbcb1e2d8d050c.1562854091.git.alifm@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>