getgroups.c: ignore 65534(nobody) in supplementary groups

[ptrcnull]

when running in some unprivileged namespaces, the supplementary groups are just all 65534(nobody) - getgroups returning nobody makes the chgrp test fail:

----- chgrp log follows
Testing for symlinks using 'test -h'
chgrp: /tmp/src/rsync-3.4.1/testtmp/chgrp/from/foo-65534: Invalid argument
Can't chgrp
----- chgrp log ends