hwmon: (acpi_power_meter) Check ACPI_COMPANION() against NULL

hwmon: (acpi_power_meter) 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
acpi_power_meter hwmon driver.

Fixes: afc6c4aedea5 ("hwmon: (acpi_power_meter) Convert ACPI driver to a platform one")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/5068745.GXAFRqVoOG@rafael.j.wysocki
Signed-off-by: Guenter Roeck <linux@roeck-us.net>