PM / sleep: fix freeze_ops NULL pointer dereferences

This patch fixes a NULL pointer dereference issue introduced by
commit 1f0b63866fc1 (ACPI / PM: Hold ACPI scan lock over the "freeze"
sleep state).

Fixes: 1f0b63866fc1 (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>

kernel/power/suspend.c

@@ -306,7 +306,7 @@ int suspend_devices_and_enter(suspend_state_t state)
 		error = suspend_ops->begin(state);
 		if (error)
 			goto Close;
-	} else if (state == PM_SUSPEND_FREEZE && freeze_ops->begin) {
+	} else if (state == PM_SUSPEND_FREEZE && freeze_ops && freeze_ops->begin) {
 		error = freeze_ops->begin();
 		if (error)
 			goto Close;
@@ -335,7 +335,7 @@ int suspend_devices_and_enter(suspend_state_t state)
  Close:
 	if (need_suspend_ops(state) && suspend_ops->end)
 		suspend_ops->end();
-	else if (state == PM_SUSPEND_FREEZE && freeze_ops->end)
+	else if (state == PM_SUSPEND_FREEZE && freeze_ops && freeze_ops->end)
 		freeze_ops->end();
 
 	return error;