mes: with-input-from-string: peek fix.

* module/mes/guile.mes (with-input-from-string): Return actual tell.
* tests/guile.test ("with-input-from-string peek"): Test it.

module/mes/guile.mes

@@ -71,8 +71,9 @@
   (let ((tell 0)
         (end (string-length string)))
     (set! peek-char
-          (lambda () (if (= tell end) (integer->char -1)
-                         (string-ref string (- tell 1)))))
+          (lambda ()
+            (if (= tell end) (integer->char -1)
+                (string-ref string tell))))
     (set! read-char
           (lambda () (if (= tell end) (integer->char -1)
                          (begin

tests/guile.test

@@ -112,4 +112,9 @@ exit $?
     (push-input sp)
     (and (pop-input) (not (pop-input)))))
 
+(pass-if-equal "with-input-from-string peek"
+               #\X
+               (with-input-from-string "X"
+                 (lambda () (peek-char))))
+
 (result 'report)