small fix

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index e0aefed..9ec861c 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -3205,7 +3205,7 @@
 
             fail_timeout = ngx_parse_time(&s, 1);
 
-            if (fail_timeout < 0) {
+            if (fail_timeout == NGX_ERROR) {
                 goto invalid;
             }