BUG/MINOR: mux-h2: always reset rcvd_s when switching to a new frame

In Patrick's trace it was visible that after a stream had been missed,
the next stream would receive a WINDOW_UPDATE with the first one's
credit added to its own. This makes sense because in case of error
h2c->rcvd_s is not reset. Given that this counter is per frame, better
reset it when starting to parse a new frame, it's easier and safer.

This must be backported as far as 1.8.

(cherry picked from commit 30d05f35571ebff61666e1954a696a25503ed6b0)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/mux_h2.c b/src/mux_h2.c
index e2867a5..984a81b 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2309,6 +2309,8 @@
 			break;
 
 		if (h2c->st0 == H2_CS_FRAME_H) {
+			h2c->rcvd_s = 0;
+
 			if (!h2_peek_frame_hdr(&h2c->dbuf, 0, &hdr))
 				break;