Upstream: avoid duplicate finalization.

Upstream: avoid duplicate finalization.

A request may be already finalized when ngx_http_upstream_finalize_request()
is called, due to filter finalization: after filter finalization upstream
can be finalized via ngx_http_upstream_cleanup(), either from
ngx_http_terminate_request(), or because a new request was initiated
to an upstream.  Then the upstream code will see an error returned from
the filter chain and will call the ngx_http_upstream_finalize_request()
function again.

To prevent corruption of various upstream data in this situation, make sure
to do nothing but merely call ngx_http_finalize_request().

Prodded by Yichun Zhang, for details see the thread at
http://nginx.org/pipermail/nginx-devel/2015-February/006539.html.