racket/gui gtk: fix event handling on choice%

Button-click handling foir `choice%` could continuously
re-queue an event that isn't handled by default, leading
to stack overflow and a crash.

pkgs/gui-pkgs/gui-lib/mred/private/wx/gtk/combo.rkt

@@ -67,11 +67,13 @@
           (or v
               (begin
                 (g_signal_handler_unblock gtk other-id)
+		(set! unblocked? #t)
                 (let ([r (g_signal_emit gtk
                                         button-press-id
                                         0
                                         event)])
                   (g_signal_handler_block gtk other-id)
+		  (set! unblocked? #f)
                   r)))))))
 
 ;; Dependence on the implemenation of GtkComboBox: