Change VG_(getgroups):
(a) it is an error to pass in a size < 0, independent of platform
(b) getgroups does not return a value > size; no need to check for that.


git-svn-id: svn://svn.valgrind.org/valgrind/branches/BUF_REMOVAL@14591 a5019735-40e9-0310-863c-91ae7b9d1cf9