_source_query_id: follow up fix for last commit

Oops, I forgot to use the newly created $t_source_query_id variable in
commit 3110481c6492ffcb9b3f2f6886897508db4ed1aa. We now no longer
directly assume $t_filter['_source_query_id'] exists and will use the
default of -1 if that filter element is undefined.