udev-rules: ignore OWNER=/GROUP= with unknown user/group

udev-rules: ignore OWNER=/GROUP= with unknown user/group

Previously, when an unknown or invalid user/group is specified,
a token was installed with UID_INVALID/GID_INVALID. That's not only
meaningless in most cases, but also clears previous assignment,
if multiple OWNER=/GROUP= token exist for the same device, e.g.

KERNEL=="sda", GROUP="disk"
KERNEL=="sda", GROUP="nonexistentuser"

This makes when an unknown user/group is specified, the line will be
ignored. Hence, in the above example, the device will be owned by the
group "disk".