device: Ensure we have sysfs path before comparing.

device: Ensure we have sysfs path before comparing.

In some cases we do not have a udev device when setting up a unit
(certainly the code gracefully handles this). However, we do
then go on to compare the path via path_equal which will assert
if a null value is passed in.

See https://bugs.mageia.org/show_bug.cgi?id=17766

Not sure if this is the correct fix, but it avoids the crash