BUG/MINOR: mux-h2: Use the dummy error when decoding headers for a closed stream

Since the commit 6884aa3e ("BUG/MAJOR: mux-h2: Handle HEADERS frames received
after a RST_STREAM frame"), HEADERS frames received for an unknown or already
closed stream are decoded. Once decoded, an error is reported for the
stream. But because it is a dummy stream (h2_closed_stream), its state cannot be
changed. So instead, we must return the dummy error stream (h2_error_stream).

This patch must be backported to 2.0 and 1.9.

(cherry picked from commit ea7a7781a94addb9fb18ef8064c96d73fe5add3d)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 708a261..8c79702 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2125,7 +2125,7 @@
 		 * the data and send another RST.
 		 */
 		error = h2c_decode_headers(h2c, &rxbuf, &flags, &body_len);
-		h2s_error(h2s, H2_ERR_STREAM_CLOSED);
+		h2s = (struct h2s*)h2_error_stream;
 		h2c->st0 = H2_CS_FRAME_E;
 		goto send_rst;
 	}