Mark some class methods as pure.
warnings look like this:
> dbaccess/source/core/api/RowSetBase.cxx
> 593    assertWithSideEffect	398	warning	Assert statement calls
> a function which may have desired side effects: 'isAfterLast'.

Change-Id: I1b69340c3714be4678b599fa9a8ca933122aa857
Reviewed-on: https://gerrit.libreoffice.org/23981Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>