Fix of cpu hog in event pipe. If client closed connection in ngx_event_pipe_write_to_downstream(), buffers in the "out" chain were lost. This caused cpu hog if all available buffers were in the "out" chain. Fix is to call ngx_chain_update_chains() before checking return code of output filter to avoid loosing buffers in the "out" chain. Note that this situation (all available buffers in the "out" chain) isn't normal, it should be prevented by busy buffers limit. Though right now it may happen with complex protocols like fastcgi. This should be addressed separately.