win32: clipboard fix

original commit: 4e7281768818743ef9c9b779affef23448dff344

collects/mred/private/wx/win32/clipboard.rkt

@@ -148,11 +148,12 @@
                            ;; no conversion:
                            d])))]
            [all-handles (for/list ([d (in-list all-data)])
+			  (and d
 			       (let ([h (GlobalAlloc GHND (bytes-length d))])
 				 (let ([p (GlobalLock h)])
 				   (memcpy p d (bytes-length d)))
 				 (GlobalUnlock h)
-                            h))])
+				 h)))])
       (if (null? types)
           (drop-client c)
           (atomically
@@ -161,7 +162,8 @@
                  (EmptyClipboard)
                  (for ([t (in-list type-ids)]
                        [h (in-list all-handles)])
-                   (SetClipboardData t h))
+		   (when h
+		     (SetClipboardData t h)))
                  (if (CloseClipboard)
                      (set! client c)
                      (drop-client c)))