Avoid an assert inside rasqal_query_results_next_triple

(librdf_query_rasqal_query_results_next_statement): check that
"result" pointer is not NULL before calling
rasqal_query_results_next_triple()

Patch from Michael Stahl <mstahl@redhat.com>

Fixes Issue #0000543
http://bugs.librdf.org/mantis/view.php?id=543