crashtesting: null deref in SwCellFrame::GetLayoutRowSpan

seen with swtextframe_connectfootnote_heap_use_after_free.sample

Change-Id: I98da2e67065c5890d74e5f33ffc01329ff75f185
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/96257
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
diff --git a/sw/source/core/layout/tabfrm.cxx b/sw/source/core/layout/tabfrm.cxx
index 0a3e5f9..2642c50 100644
--- a/sw/source/core/layout/tabfrm.cxx
+++ b/sw/source/core/layout/tabfrm.cxx
@@ -5366,7 +5366,8 @@ void SwCellFrame::Modify( const SfxPoolItem* pOld, const SfxPoolItem * pNew )

long SwCellFrame::GetLayoutRowSpan() const
{
    long nRet = GetTabBox()->getRowSpan();
    const SwTableBox *pTabBox = GetTabBox();
    long nRet = pTabBox ? pTabBox->getRowSpan() : 0;
    if ( nRet < 1 )
    {
        const SwFrame* pRow = GetUpper();