BUG/MINOR: mux-fcgi: Handle empty STDERR record

BUG/MINOR: mux-fcgi: Handle empty STDERR record

As mentionned in the FastCGI specification, FCGI "streams" are series of
non-empty stream records (length != 0), followed by an empty one. It is properly
handled for FCGI_STDOUT records, but not for FCGI_STDERR ones. If an empty
FCGI_STDERR record is received, the connection is blocked waiting for data which
will never come.

To fix the bug, when an empty FCGI_STDERR record is received, we drop it, eating
the padding if any.

This patch should fix the issue #743. It must be backported as far as 2.1.

(cherry picked from commit 7f85433a912529c5cda1629001b34fd2b2e54758)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit bd6dc07038f4c6b552559ccde41d31ff9a7dbf6d)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>