7043788: (fs) PosixFileAttributes.owner() or group() throws NPE if not group in passwd/group database

[Alan Bateman]

This is another little embarrassing one. A test that reads the 
PosixFileAttributes and prints out the owner and group-owner fails with 
NPE if the the lookup of the uid or gid fails because it's not in the 
passwd or group database. It turns out that the "not found" handling in 
the native code isn't correct with the result that it returns null for 
these cases instead of failing. The webrev with the changes is here:

http://cr.openjdk.java.net/~alanb/7043788/webrev/

I've decided not to include a test for this because it requires 
privileges to chown or chgrp a file to some unknown uid/gid.

Thanks,

-Alan