rasqal_rowsource_copy_variables add debug error report
Fix variables count for SPARQL XML results

(rasqal_service_rowsource_ensure_variables): Copy variables in

(rasqal_sparql_xml_sax2_end_element_handler): Take variables count from
rowsource size, not vars_table size.

Fixes Issue #0000588
http://bugs.librdf.org/mantis/view.php?id=588