hwmon: Fix possible NULL pointer

The recent addition of the label attribute added some code that read the
"label" device property, without checking first that "dev" was non-NULL.

Fix this issue by first checking that "dev" is non-NULL.

Fixes: ccd98cba6a18 ("hwmon: Add "label" attribute")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/hwmon.c

@@ -809,7 +809,7 @@ __hwmon_device_register(struct device *dev, const char *name, void *drvdata,
 		hdev->groups = groups;
 	}
 
-	if (device_property_present(dev, "label")) {
+	if (dev && device_property_present(dev, "label")) {
 		err = device_property_read_string(dev, "label", &label);
 		if (err < 0)
 			goto free_hwmon;