fix: allow std.os.linux.getgroups to accept null

[sweetbbak]

looking at man getgroups and info getgroups this is given as an example:

     // Here's how to use ‘getgroups’ to read all the supplementary group
     // IDs:

          gid_t *
          read_all_groups (void)
          {
            int ngroups = getgroups (0, NULL);
            gid_t *groups
              = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
            int val = getgroups (ngroups, groups);
            if (val < 0)
              {
                free (groups);
                return NULL;
              }
            return groups;
          }

getgroups(0, NULL) is used to get the count of groups so that the correct count can be used to allocate a list of gid_t. However this is not possible with the function signature getgroups(size: usize, list: *gid_t) usize where "list" is supposedly a pointer to the beginning of a list of unknown length.

equivalent example in Zig after the change:

  // get the group count
  const ngroups: usize = std.os.linux.getgroups(0, null);
  if (ngroups <= 0) {
      return error.GetGroupsError;
  }

  std.debug.print("number of groups: {d}\n", .{ngroups});
  const groups_gids: []u32 = try alloc.alloc(u32, ngroups);

  // populate an array of gid_t
  _ = std.os.linux.getgroups(ngroups, @ptrCast(groups_gids));