SwFrm::GetBackgroundBrush(): silence assert

getSdrAllFillAttributesHelper() should be called on an SwFrm in case
supportsFullDrawingLayerFillAttributeSet() retruns true, otherwise we
get:

getSdrAllFillAttributesHelper() call only valid for RES_FLYFRMFMT and RES_FRMFMT (!)

Change-Id: I08b5bffee4f2ebd852017640f9c2e5a5075ca2ec

sw/source/core/layout/paintfrm.cxx

@@ -7477,7 +7477,8 @@ bool SwFrm::GetBackgroundBrush(
             return false;
 
         //UUUU
-        rFillAttributes = pFrm->getSdrAllFillAttributesHelper();
+        if (pFrm->supportsFullDrawingLayerFillAttributeSet())
+            rFillAttributes = pFrm->getSdrAllFillAttributesHelper();
         const SvxBrushItem &rBack = pFrm->GetAttrSet()->GetBackground();
 
         if( pFrm->IsSctFrm() )