intel-iommu: Fix oops in device_to_iommu() when devices not found.

It's possible for a device in the drhd->devices[] array to be NULL if
it wasn't found at boot time, which means we have to check for that
case.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/pci/intel-iommu.c

@@ -485,7 +485,8 @@ static struct intel_iommu *device_to_iommu(int segment, u8 bus, u8 devfn)
 			    drhd->devices[i]->bus->number == bus &&
 			    drhd->devices[i]->devfn == devfn)
 				return drhd->iommu;
-			if (drhd->devices[i]->subordinate &&
+			if (drhd->devices[i] &&
+			    drhd->devices[i]->subordinate &&
 			    drhd->devices[i]->subordinate->number <= bus &&
 			    drhd->devices[i]->subordinate->subordinate >= bus)
 				return drhd->iommu;