ACPI: PAD: xen: Check ACPI_COMPANION() against NULL

ACPI: PAD: xen: Check ACPI_COMPANION() against NULL

Every platform driver can be forced to match a device that doesn't match
its list of device IDs because of device_match_driver_override(), so
platform drivers that rely on the existence of a device's ACPI companion
object need to verify its presence.

Accordingly, add a requisite ACPI_COMPANION() check against NULL to the
Xen variant of the ACPI processor aggregator device (PAD) driver.

Fixes: 112b2f978afe ("ACPI: PAD: xen: Convert to a platform driver")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Juergen Gross <jgross@suse.com>
Link: https://patch.msgid.link/3427762.aeNJFYEL58@rafael.j.wysocki