Upstream: reject upstreams without normal servers.

Such upstreams cause CPU hog later in the code as number of peers isn't
expected to be 0.  Currently this may happen either if there are only backup
servers defined in an upstream block, or if server with ipv6 address used
in an upstream block.

src/http/ngx_http_upstream_round_robin.c

@@ -49,6 +49,13 @@ ngx_http_upstream_init_round_robin(ngx_conf_t *cf,
             n += server[i].naddrs;
         }
 
+        if (n == 0) {
+            ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
+                          "no servers in upstream \"%V\" in %s:%ui",
+                          &us->host, us->file_name, us->line);
+            return NGX_ERROR;
+        }
+
         peers = ngx_pcalloc(cf->pool, sizeof(ngx_http_upstream_rr_peers_t)
                               + sizeof(ngx_http_upstream_rr_peer_t) * (n - 1));
         if (peers == NULL) {