bug#25267: guile-2.2 crash in GC

[Linas Vepstas <linasvepstas@gmail.com>]

FYI, this is quickly and easily reproducible, happens within seconds,
and hits the same spot every time. Note-to-self (not for general
consumption): my unit test to provoke this is to start the cogserver
and run this shell script:

#!/bin/bash

i=0
while true ; do
  let i=$i+1
  if [ "$(($i % 2000))" -eq "0" ] ; then
    echo loop $i
  fi
  # echo '(display ctr)' | nc localhost 17001
  echo '(NumberNode ctr)' | nc localhost 17001
done

other testing variants are described in
https://github.com/opencog/opencog/issues/2550