scheme/foreign: allow #f as NULL function pointer, allow callback as cpointer

svn: r12992

original commit: d596401804242ca39007e56d798e12f3852810f0

collects/mzlib/foreign.ss

@@ -477,6 +477,7 @@
   (define-syntax-rule (make-it wrap)
     (make-ctype _fpointer
       (lambda (x)
+        (and x
              (let ([cb (ffi-callback (wrap x) itypes otype abi)])
                (cond [(eq? keep #t) (hash-set! held-callbacks x cb)]
                      [(box? keep)
@@ -484,8 +485,8 @@
                         (set-box! keep
                                   (if (or (null? x) (pair? x)) (cons cb x) cb)))]
                      [(procedure? keep) (keep cb)])
-          cb))
-      (lambda (x) (wrap (ffi-call x itypes otype abi)))))
+               cb)))
+      (lambda (x) (and x (wrap (ffi-call x itypes otype abi))))))
   (if wrapper (make-it wrapper) (make-it begin)))
 
 ;; Syntax for the special _fun type: