BUG/MINOR: server: fix down_time report for stats

Adjust condition used to report down_time for statistics. There was a
tiny probabilty to have a negative downtime if last_change was superior
to now. If this is the case, return only down_time.

This bug can backported up to 1.8.
diff --git a/src/server.c b/src/server.c
index c757cdd..5ea6ca1 100644
--- a/src/server.c
+++ b/src/server.c
@@ -74,7 +74,7 @@
 
 int srv_downtime(const struct server *s)
 {
-	if ((s->cur_state != SRV_ST_STOPPED) && s->last_change < now.tv_sec)		// ignore negative time
+	if ((s->cur_state != SRV_ST_STOPPED) || s->last_change >= now.tv_sec)		// ignore negative time
 		return s->down_time;
 
 	return now.tv_sec - s->last_change + s->down_time;