Event pipe: fixed buffer loss in p->length case. With previous code raw buffer might be lost if p->input_filter() was called on a buffer without any data and used ngx_event_pipe_add_free_buf() to return it to the free list. This eventually might cause "all buffers busy" problem, resulting in segmentation fault due to null pointer dereference in ngx_event_pipe_write_chain_to_temp_file(). In ngx_event_pipe_add_free_buf() the buffer was added to the list start due to pos == last, and then "p->free_raw_bufs = cl->next" in ngx_event_pipe_read_upstream() dropped both chain links to the buffer from the p->free_raw_bufs list. Fix is to move "p->free_raw_bufs = cl->next" before calling the p->input_filter().