on examination this PreparePaint virtual is only called from
SvTreeListBox::PaintEntry1

and PaintEntry1 is only called from SvImpLBox::Paint in a

    for(sal_uInt16 n=0; n< nCount && pEntry; n++)
    {
        /*long nMaxRight=*/
        pView->PaintEntry1

loop so pEntry always exists given that test. Re-jig things
so these families of method take a reference instead of a pointer
so verifying it cannot be NULL and a whole pile of else paths
fall away

Change-Id: Ied40acb1c2263c21b4447832f8cb86f64ed9e80d