FastCGI: fixed wrong connection close with fastcgi_keep_conn.

FastCGI: fixed wrong connection close with fastcgi_keep_conn.

With fastcgi_keep_conn it was possible that connection was closed after
FCGI_STDERR record with zero padding and without any further data read yet.
This happended as f->state was set to ngx_http_fastcgi_st_padding and then
"break" happened, resulting in p->length being set to f->padding, i.e. 0
(which in turn resulted in connection close).

Fix is to make sure we continue the loop after f->state is set.