ACPI / fan: avoid null pointer deference error

Fix a null pointer deference by acpi_driver_data() if device is
null.  We should only set cdev and check this is OK after we are
sure device is not null.

Smatch analysis:

drivers/acpi/fan.c:179 acpi_fan_remove() warn: variable dereferenced
  before check 'device' (see line 177)
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/fan.c

@@ -174,9 +174,13 @@ static int acpi_fan_add(struct acpi_device *device)
 
 static int acpi_fan_remove(struct acpi_device *device)
 {
-	struct thermal_cooling_device *cdev = acpi_driver_data(device);
+	struct thermal_cooling_device *cdev;
+
+	if (!device)
+		return -EINVAL;
 
-	if (!device || !cdev)
+	cdev =  acpi_driver_data(device);
+	if (!cdev)
 		return -EINVAL;
 
 	sysfs_remove_link(&device->dev.kobj, "thermal_cooling");