bugfix: body_filter_by_lua*: we might incorrectly pass zero-size bufs (in the form of "special sync bufs") at the beginning of a chain, which could get stuck in the buffer of ngx_http_writer_filter_module (or in other words, being "busy") while could still get recycled in the content handler (like content_by_lua), leading to buffer corruptions. thanks westhood for the report and patch in #386.