usb-redir: fix use-after-free

Reinitialize dev->cs to NULL after deleting it, to make sure it isn't
used afterwards.
Reported-by: NMartin Cerveny <M.Cerveny@computer.org>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/redirect.c

@@ -1334,6 +1334,7 @@ static void usbredir_handle_destroy(USBDevice *udev)
     USBRedirDevice *dev = DO_UPCAST(USBRedirDevice, dev, udev);
 
     qemu_chr_delete(dev->cs);
+    dev->cs = NULL;
     /* Note must be done after qemu_chr_close, as that causes a close event */
     qemu_bh_delete(dev->chardev_close_bh);