If a Mac user has more than 256 groups, getGroupList returns -1 but
does not correctly set n. We need to retry the syscall with an
ever-increasing group size until we get all of the user's groups.
The easiest way to test this change is to set n to a value lower than
the current user's number of groups, test on a Mac and observe
a failure, then apply the patch and test that it passes.