Fix getgroups() with zero size parameter (Kirill Shutemov).

According to man page getgroups(2):

If size is zero, list is not modified, but the total number of
supplementary group IDs for the process is returned.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5266 c046a42c-6fe2-441c-8c8c-71466251a162