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.