[BUG] Restore info about available active/backup servers

Bug introduced by 5f5b7d2c1db1fabe3d372b0080a8773147762ee7

This bug was reported by Guido Krause.
(cherry picked from commit 0c801d1f5ffdc2fe3d036c1e5203d617507c55c3)
diff --git a/src/checks.c b/src/checks.c
index d326625..ef7839b 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -170,7 +170,7 @@
 			chunk_printf(msg, ", check duration: %ldms", s->check_duration);
 	}
 
-	if (xferred > 0) {
+	if (xferred >= 0) {
 		if (!(s->state & SRV_RUNNING))
         	        chunk_printf(msg, ". %d active and %d backup servers left.%s"
 				" %d sessions active, %d requeued, %d remaining in queue",