BUG/MINOR: server: fix srv downtime calcul on starting

When a server is up after a failure, its downtime was reset to 0 on the
statistics. This is due to a wrong condition that causes srv.down_time
to never be set. Fix this by updating down_time each time the server is in
STARTING state.

Fixes the github issue #920.
This bug can be backported up to 1.8.
diff --git a/src/server.c b/src/server.c
index bcef94f..c757cdd 100644
--- a/src/server.c
+++ b/src/server.c
@@ -4729,7 +4729,7 @@
 				s->proxy->last_change = now.tv_sec;
 			}
 
-			if (s->next_state == SRV_ST_STOPPED && s->last_change < now.tv_sec)	// ignore negative times
+			if (s->cur_state == SRV_ST_STOPPED && s->last_change < now.tv_sec)	// ignore negative times
 				s->down_time += now.tv_sec - s->last_change;
 
 			s->last_change = now.tv_sec;