Fix final export-timespan count

ExportHandler::start_timespan() is also used to stop export
when there are no more timespans to be processed.

This is done because freewheeling has to be stopped from
outside the process cycle.

libs/ardour/export_handler.cc

@@ -170,8 +170,6 @@ ExportHandler::do_export ()
 int
 ExportHandler::start_timespan ()
 {
-	export_status->timespan++;
-
 	/* stop freewheeling and wait for latency callbacks */
 	if (AudioEngine::instance()->freewheeling ()) {
 		AudioEngine::instance()->freewheel (false);
@@ -186,6 +184,8 @@ ExportHandler::start_timespan ()
 		return -1;
 	}
 
+	export_status->timespan++;
+
 	/* finish_timespan pops the config_map entry that has been done, so
 	   this is the timespan to do this time
 	*/