The rare crashes in MenuFloatingWindow::ImplGetStartY() and
MenuFloatingWindow::ImplScroll(bool) likely happen because
of a disposed Menu.

Let's guard against invalid accesses.

Change-Id: Ie31240abbc48c06edd40d0a95f319725cdb3db16
Reviewed-on: https://gerrit.libreoffice.org/36026Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>