Fix a11y crash with drag&drop shape anchored to page

Dereferencing an empty boost::optional is dereferencing a this
nullptr, simply copy it instead.

Regression from

commit 0a18c1dc
CommitDate: Fri Dec 21 18:52:31 2018 +0100

    pass ScAddress around using boost::optional

Change-Id: Id676b2d84d324b989259688069d4860de5559ebc
Reviewed-on: https://gerrit.libreoffice.org/72605Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

sc/source/ui/Accessibility/AccessibleDocument.cxx

@@ -1140,7 +1140,7 @@ void ScChildrenShapes::SetAnchor(const uno::Reference<drawing::XShape>& xShape,
         if ((xAddress && pData->xRelationCell && (*xAddress != *(pData->xRelationCell))) ||
             (!xAddress && pData->xRelationCell) || (xAddress && !pData->xRelationCell))
         {
-            pData->xRelationCell = *xAddress;
+            pData->xRelationCell = xAddress;
             if (pData->pAccShape.is())
                 pData->pAccShape->SetRelationSet(GetRelationSet(pData));
         }