set absolute weight, this fixes bogus "no live upstream" case when
last upstream is down while live one has negative weight

src/http/ngx_http_upstream_round_robin.c

diff --git a/src/http/ngx_http_upstream_round_robin.c b/src/http/ngx_http_upstream_round_robin.c
index 79747f6..c25c076 100644
--- a/src/http/ngx_http_upstream_round_robin.c
+++ b/src/http/ngx_http_upstream_round_robin.c
@@ -605,7 +605,7 @@
         }
 
         for (i = 0; i < peers->number; i++) {
-            peer[i].current_weight += peer[i].weight;
+            peer[i].current_weight = peer[i].weight;
         }
     }
 }