Gracefully handle the case where the token stream just contains the END token and an error occurs.

original commit: 69ae77a253fadbcb4ad4e87514e18f2a01b97068

collects/algol60/cfg-parser.rkt

@@ -744,6 +744,18 @@
                                    val
                                    (next success-k fail-k max-depth tasks)))]
                             [fail-k (lambda (max-depth tasks)
+                                      (cond
+                                       [(null? tok-list)
+                                        (if error-proc
+                                            (error-proc #t
+                                                        'no-tokens
+                                                        #f
+                                                        (make-position #f #f #f)
+                                                        (make-position #f #f #f))
+                                            (error
+                                             'cfg-parse
+                                             "no tokens"))]
+                                       [else
                                         (let ([bad-tok (list-ref tok-list 
                                                                  (min (sub1 (length tok-list))
                                                                       max-depth))])
@@ -756,7 +768,7 @@
                                               (error
                                                'cfg-parse
                                                "failed at ~a" 
-                                             (tok-val bad-tok)))))])
+                                               (tok-val bad-tok))))]))])
                      (#,start tok-list
                               ;; we simulate a token at the very beginning with zero width
                               ;; for use with the position-generating code (*-start-pos, *-end-pos).