intel_iommu: a fix to vtd_find_as_from_bus_num()

intel_iommu: a fix to vtd_find_as_from_bus_num()

Ensure the return value of vtd_find_as_from_bus_num() is NULL by
enforcing vtd_bus=NULL. This would help caller of vtd_find_as_from_bus_num()
to decide if any further operation on the returned vtd_bus.

Cc: qemu-stable@nongnu.org
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Yi Sun <yi.y.sun@linux.intel.com>
Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
Signed-off-by: Yi Sun <yi.y.sun@linux.intel.com>
Message-Id: <1578058086-4288-2-git-send-email-yi.l.liu@intel.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit a2e1cd41ccfe796529abfd1b6aeb1dd4393762a2)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>