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.
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index efe5d54..571401d 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -2336,7 +2336,7 @@
 		return 0;
 	}
 	fconn->state = FCGI_CS_RECORD_H;
-	fstrm->state |= FCGI_SF_ES_RCVD;
+	fstrm->flags |= FCGI_SF_ES_RCVD;
 	TRACE_PROTO("FCGI STDOUT record rcvd", FCGI_EV_RX_RECORD|FCGI_EV_RX_STDOUT, fconn->conn, fstrm,, (size_t[]){0});
 	TRACE_STATE("stdout data fully send, switching to RECORD_H", FCGI_EV_RX_RECORD|FCGI_EV_RX_FHDR|FCGI_EV_RX_EOI, fconn->conn, fstrm);
 	TRACE_LEAVE(FCGI_EV_RX_RECORD|FCGI_EV_RX_STDOUT, fconn->conn, fstrm);