It was an undefined behavior that static_cast'ing a pointer to
SmVisibleNode to SmStructureNode's.

Change-Id: Ic391175a5535cec84bd7f9ec58fa13a411398331
Reviewed-on: https://gerrit.libreoffice.org/26676Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>