bool ScDocShell::isTiledRendering() const crashed without this check

Change-Id: Idb7ace05eb1aa37cbc3d956503c046b0e9b9d0e5
Reviewed-on: https://gerrit.libreoffice.org/19570Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>