Ensured that rdf_writer_for() calls abort() on never-reached branches.
Ensured that rdf_writer_for() calls abort() on never-reached branches.

dirvine/librdf,bxshi/librdf,bxshi/librdf,dirvine/librdf,datagraph/librdf,dirvine/librdf,bxshi/librdf,datagraph/librdf,datagraph/librdf