Verify parent_dev before calling udev_device_get_sysattr_value

Not all udev devices have parent devices.
Calling udev_device_get_ functions yield an assertion error
if called with a NULL pointer.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Sietse <sietse@wizdom.nu>
Co-authored-by: Sietse <sietse@wizdom.nu>
Closes openzfs#16705 
Closes openzfs#16717

cmd/zed/zed_disk_event.c

@@ -139,7 +139,8 @@ dev_event_nvlist(struct udev_device *dev)
 		 * is /dev/sda.
 		 */
 		struct udev_device *parent_dev = udev_device_get_parent(dev);
-		if ((value = udev_device_get_sysattr_value(parent_dev, "size"))
+		if (parent_dev != NULL &&
+		    (value = udev_device_get_sysattr_value(parent_dev, "size"))
 		    != NULL) {
 			uint64_t numval = DEV_BSIZE;