PM / sleep: fix freeze_ops NULL pointer dereferences

This patch fixes a NULL pointer dereference issue introduced by
commit 1f0b6386 (ACPI / PM: Hold ACPI scan lock over the "freeze"
sleep state).

Fixes: 1f0b6386 (ACPI / PM: Hold ACPI scan lock over the "freeze" sleep state)
Link: http://marc.info/?l=linux-pm&m=140541182017443&w=2


Reported-and-tested-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>