fix segfault if there is error_page 401, proxy_intercept_errors is on
and backend does not return "WWW-Authenticate" header

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index edd48ac..16a027a 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -1606,8 +1606,9 @@
 
         if (err_page[i].status == status) {
 
-            if (status == NGX_HTTP_UNAUTHORIZED) {
-
+            if (status == NGX_HTTP_UNAUTHORIZED
+                && u->headers_in.www_authenticate)
+            {
                 h = ngx_list_push(&r->headers_out.headers);
 
                 if (h == NULL) {