While uid_t and gid_t are both unsigned, the values (uid_t) -1 and
(gid_t) -1 are reserved.  A uid or gid argument of -1 to the chown(2)
system call means to leave the uid/gid unchanged.  Catch this case
so that trying to set a uid or gid to -1 will result in an error.

Test cases:

  chown 4294967295 file
  chown :4294967295 file
  chgrp 4294967295 file

Andreas Gruenbacher <agruen@suse.de>

---
 src/chgrp.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: src/chgrp.c
===================================================================
--- src/chgrp.c.orig	2014-01-03 18:18:24.257902746 +0100
+++ src/chgrp.c	2014-01-03 19:47:23.019440368 +0100
@@ -88,7 +88,7 @@ parse_group (const char *name)
         {
           unsigned long int tmp;
           if (! (xstrtoul (name, NULL, 10, &tmp, "") == LONGINT_OK
-                 && tmp <= GID_T_MAX))
+                 && tmp <= GID_T_MAX && (gid_t) tmp != (gid_t) -1))
             error (EXIT_FAILURE, 0, _("invalid group: %s"), quote (name));
           gid = tmp;
         }