Upstream: fixed "zero size buf" alerts with cache (ticket #918).

Upstream: fixed "zero size buf" alerts with cache (ticket #918).

If caching was used, "zero size buf in output" alerts might appear
in logs if a client prematurely closed connection.  Alerts appeared
in the following situation:

- writing to client returned an error, so event pipe
  drained all busy buffers leaving body output filters
  in an invalid state;

- when upstream response was fully received,
  ngx_http_upstream_finalize_request() tried to flush
  all pending data.

Fix is to avoid flushing body if p->downstream_error is set.