BUG/MINOR: mux-fcgi: Set flags on the right stream field for empty FCGI_STDOUT

BUG/MINOR: mux-fcgi: Set flags on the right stream field for empty FCGI_STDOUT

In fcgi_strm_handle_empty_stdout(), the FCGI_SF_ES_RCVD flag is set on "->state"
stream field instead of "->flags". It is obviously wrong. This bug is not
noticeable because the right state is set in the fcgi_process_demux() function a
bit later.

This patch must be backported as far as 2.1.

(cherry picked from commit 3b3096ede1b52007fa49a563436df8ee9323d78c)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit ca5e8c058e157ee7d549014705e23e3be6fc505c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>