_source_query_id: follow up fix for last commit

Oops, I forgot to use the newly created $t_source_query_id variable in
commit 3110481. 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.

core/filter_api.php

@@ -3435,7 +3435,7 @@ function <?php echo $t_js_toggle_func;?>() {
 					$t_source_query_id = isset( $t_filter['_source_query_id'] ) ? $t_filter['_source_query_id'] : -1;
 					foreach( $t_stored_queries_arr as $t_query_id => $t_query_name ) {
 						echo '<option value="' . $t_query_id . '" ';
-						check_selected( $t_query_id, $t_filter['_source_query_id'] );
+						check_selected( $t_query_id, $t_source_query_id );
 						echo '>' . string_display_line( $t_query_name ) . '</option>';
 					}
 					?>