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

src/core/device.c

@@ -318,7 +318,7 @@ static int device_setup_unit(Manager *m, struct udev_device *dev, const char *pa
          * the GC to have garbaged it. That's desired since the device
          * unit may have a dependency on the mount unit which was
          * added during the loading of the later. */
-        if (u && DEVICE(u)->state == DEVICE_PLUGGED) {
+        if (sysfs && u && DEVICE(u)->state == DEVICE_PLUGGED) {
                 /* This unit is in plugged state: we're sure it's
                  * attached to a device. */
                 if (!path_equal(DEVICE(u)->sysfs, sysfs)) {