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