passthrough: fix domain ref leak after 24888:71159fb049f2

As pointed out by Keir.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/passthrough/iommu.c
+++ b/xen/drivers/passthrough/iommu.c
@@ -580,6 +580,8 @@ int iommu_do_domctl(
             printk(XENLOG_G_ERR
                    "XEN_DOMCTL_assign_device: get_domain_by_id() failed\n");
             ret = -EINVAL;
+            if ( d )
+                goto assign_device_out;
             break;
         }