fstab-generator: fix checking of fstab_node_to_udev_node() result

fstab-generator: fix checking of fstab_node_to_udev_node() result

We have to check for OOM here, let's add that. There's really no point in
checking for path_is_absolute() on the result however, as there's no particular
reason why that should be refused. Also, we don't have similar checks for the
other mount devices the generator deals with, hence don't bother with it here
either. Let's remove that check.

(And it shouldn't return made-up errors like "-1" in this case anyway.)