Prevent oops at boot with VT-d

With some broken BIOSs when VT-d is enabled, the data structures are
filled incorrectly. This can cause a NULL pointer dereference in very
early boot.
Signed-off-by: NDirk Hohndel <hohndel@linux.intel.com>
Acked-by: NYu Zhao <yu.zhao@intel.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/pci/intel-iommu.c

@@ -438,7 +438,8 @@ static struct intel_iommu *device_to_iommu(u8 bus, u8 devfn)
 			continue;
 
 		for (i = 0; i < drhd->devices_cnt; i++)
-			if (drhd->devices[i]->bus->number == bus &&
+			if (drhd->devices[i] &&
+			    drhd->devices[i]->bus->number == bus &&
 			    drhd->devices[i]->devfn == devfn)
 				return drhd->iommu;