FastCGI: skip special bufs in buffered request body chain. This prevents forming empty records out of such buffers. Particularly it fixes double end-of-stream records with chunked transfer encoding, or when HTTP/2 is used and the END_STREAM flag has been sent without data. In both cases there is an empty buffer at the end of the request body chain with the "last_buf" flag set. The canonical libfcgi, as well as php implementation, tolerates such records, while the HHVM parser is more strict and drops the connection (ticket #950).