udev: fix segfault with android rndis

The android gadget driver for network tethering over rndis somehow has a
parent device with a null subsystem.  Probably this is bug in android driver,
but it is easy enough to make systemd/udev behave gracefully and not
segfault.  And this will help for making linux distros with systemd
(like fedora) work on android devices.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>