ACPI: processor: Fix NULL-pointer dereference in acpi_processor_errata_piix4()

In the Linux kernel, the following vulnerability has been resolved:

ACPI: processor: Fix NULL-pointer dereference in acpi_processor_errata_piix4()

In acpi_processor_errata_piix4(), the pointer dev is first assigned an IDE

device and then reassigned an ISA device:

dev = pci_get_subsys(..., PCI_DEVICE_ID_INTEL_82371AB, ...);

dev = pci_get_subsys(..., PCI_DEVICE_ID_INTEL_82371AB_0, ...);

If the first lookup succeeds but the second fails, dev becomes NULL. This

leads to a potential null-pointer dereference when dev_dbg() is called:

if (errata.piix4.bmisx)

dev_dbg(&dev->dev, ...);

To prevent this, use two temporary pointers and retrieve each device

independently, avoiding overwriting dev with a possible NULL value.

[ rjw: Subject adjustment, added an empty code line ]