Bug in credentials handling.

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=2830

Reported by @grubba

$ cat bug.cred.pike class User{}

object luser = User();

int main(int argc, array(string) argv)
{
  object luser_creds = Pike.Security.Creds(luser, 0, 0);

  mixed result = call_with_creds(luser_creds, Stdio.File, "/dev/null");

  return 0;
}
$ ./pike -mmaster.pike bug.cred.pike
Segmentation Fault (core dumped)