BUG/MINOR: mux-h2: always send stream window update before connection's

In h2_process_mux() if we have some room and an attempt to send a window
update for the connection was pending, it's done first. But it's not done
for the stream, which will have for effect of postponing this attempt
till next pass into h2_process_demux(), at the risk of seeing the send
buffer full again. Let's always try to send both pending frames as soon
as possible.

This should be backported as far as 1.8.

(cherry picked from commit e74679a9c65009931c75ca92e31751b2a5573554)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/mux_h2.c b/src/mux_h2.c
index c2a8406..e2867a5 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2671,6 +2671,11 @@
 	}
 
 	/* start by sending possibly pending window updates */
+	if (h2c->rcvd_s > 0 &&
+	    !(h2c->flags & (H2_CF_MUX_MFULL | H2_CF_MUX_MALLOC)) &&
+	    h2c_send_strm_wu(h2c) < 0)
+		goto fail;
+
 	if (h2c->rcvd_c > 0 &&
 	    !(h2c->flags & (H2_CF_MUX_MFULL | H2_CF_MUX_MALLOC)) &&
 	    h2c_send_conn_wu(h2c) < 0)