bug#25267: crashes here only for invalid scheme

[Linas Vepstas <linasvepstas@gmail.com>]

FYI: important note: this crashes only because an exception path is
taken. Due to a "bug" in the shell script above, `ctr` is undefined,
so an unbound-variable exception is thrown.  When the scheme is valid,
then it does NOT crash here!

--linas


opencog> (NumberNode ctr)
Entering scheme shell; use ^D or a single . on a line by itself to exit.
guile> Backtrace:
In ice-9/boot-9.scm:
 157: 12 [catch #t #<catch-closure f83bce0> ...]
In unknown file:
   ?: 11 [apply-smob/1 #<catch-closure f83bce0>]
In ice-9/boot-9.scm:
 157: 10 [catch #t #<catch-closure f83b5c0> ...]
In unknown file:
   ?: 9 [apply-smob/1 #<catch-closure f83b5c0>]
   ?: 8 [call-with-input-string "(NumberNode ctr)\n" ...]
In ice-9/boot-9.scm:
2320: 7 [save-module-excursion #<procedure f818930 at
ice-9/eval-string.scm:65:9 ()>]
In ice-9/eval-string.scm:
  44: 6 [read-and-eval #<input: string f8049c0> #:lang ...]
  37: 5 [lp (NumberNode ctr)]
In ice-9/eval.scm:
 387: 4 [eval # ()]
 393: 3 [eval #<memoized ctr> ()]
In unknown file:
   ?: 2 [memoize-variable-access! #<memoized ctr> #<directory
(guile-user) bb3c60>]
In ice-9/boot-9.scm:
 102: 1 [#<procedure f181e80 at ice-9/boot-9.scm:97:6 (thrown-k .
args)> unbound-variable ...]
In unknown file:
   ?: 0 [apply-smob/1 #<catch-closure f83b580> unbound-variable ...]

ERROR: In procedure apply-smob/1:
ERROR: Unbound variable: ctr
ABORT: unbound-variable