Gzip filter: handling of empty flush buffers.

Empty flush buffers are legitimate and may happen e.g. due to $r->flush()
calls in embedded perl.  If there are no data buffered in zlib, deflate()
will return Z_BUF_ERROR (i.e. no progress possible) without adding anything
to output.  Don't treat Z_BUF_ERROR as fatal and correctly send empty flush
buffer if we have no data in output at all.

See this thread for details:
http://mailman.nginx.org/pipermail/nginx/2010-November/023693.html