fix promise bug

fix the second one in https://github.com/picrin-scheme/picrin/issues/339

contrib/20.r7rs/scheme/lazy.scm

@@ -24,9 +24,12 @@
     (if (promise-done? promise)
 	(promise-value promise)
 	(let ((new-promise ((promise-value promise))))
-          (set-promise-done!  promise (promise-done? new-promise))
-          (set-promise-value! promise (promise-value new-promise))
-          (force promise))))
+	  (if (promise-done? promise)
+	      (promise-value promise)
+	      (begin
+		(set-promise-done!  promise (promise-done? new-promise))
+		(set-promise-value! promise (promise-value new-promise))
+		(force promise))))))
 
   (define (make-promise obj)
     (if (promise? obj)

contrib/20.r7rs/t/r7rs.scm

@@ -363,7 +363,15 @@
         (force x)
         (promise? x)))
 
-
+(let ()
+  (define flag #f)
+  (define p (delay (if flag
+		       'ok
+		       (begin
+			 (set! flag #t)
+			 (force p)
+			 'ng))))
+  (test 'ok (force p)))
 
 
 (define radix