Upstream: fixed X-Accel-Redirect handling from cache files.

Upstream: fixed X-Accel-Redirect handling from cache files.

The X-Accel-Redirect header might appear in cache files if its handling
is ignored with the "proxy_ignore_headers" directive.  If the cache file
is later served with different settings, ngx_http_upstream_process_headers()
used to call ngx_http_upstream_finalize_request(NGX_DECLINED), which
is not expected to happen before the cleanup handler is installed and
resulted in ngx_http_finalize_request(NGX_DONE) (after 2b032e8df,
nginx 1.7.11), leading to unexpected request counter decrement, "request
count is zero" alerts, and segmentation faults.

Similarly, errors in ngx_http_upstream_process_headers() resulted in
ngx_http_upstream_finalize_request(NGX_HTTP_INTERNAL_SERVER_ERROR) being
called.  This is also not expected to happen before the cleanup handler is
installed, and resulted in ngx_http_finalize_request(NGX_DONE) without
proper request finalization.

Fix is to avoid calling ngx_http_upstream_finalize_request() from
ngx_http_upstream_process_headers(), notably when the cleanup handler
is not yet installed.  Errors are now simply return NGX_ERROR, so the
caller is responsible for proper finalization by calling either
ngx_http_finalize_request() or ngx_http_upstream_finalize_request().
And X-Accel-Redirect handling now does not call
ngx_http_upstream_finalize_request(NGX_DECLINED) if no cleanup handler
is installed.

Reported by Jiří Setnička
(https://mailman.nginx.org/pipermail/nginx-devel/2024-February/HWLYHOO3DDB3XTFT6X3GRMXIEJ3SJRUA.html).