FastCGI: fixed zero size buf alerts on extra data (ticket #2018).
FastCGI: fixed zero size buf alerts on extra data (ticket #2018).

After 05e42236e95b (1.19.1) responses with extra data might result in
zero size buffers being generated and "zero size buf" alerts in writer
(if f->rest happened to be 0 when processing additional stdout data).

hy0kl/nginx,hy0kl/nginx,hy0kl/nginx