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: [email protected]
Cc: Kevin Tian <[email protected]>
Cc: Jacob Pan <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Yi Sun <[email protected]>
Signed-off-by: Liu Yi L <[email protected]>
Signed-off-by: Yi Sun <[email protected]>
Message-Id: <1578058086[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>