Fix a bug where XU_NGROUPS + 1 groups might be copied.

r361780 fixed the code so that it would only remove the duplicate when
it actually existed. However, that might have resulted in XU_NGROUPS + 1
groups being copied, running off the end of the array. This patch fixes
the problem.

Spotted during code inspection for other mountd changes.

MFC after: 2 weeks