BUG/MEDIUM: mux-h2: Refuse interim responses with end-stream flag set

As state in RFC9113#8.1, HEADERS frame with the ES flag set that carries an
informational status code is malformed. However, there is no test on this
condition.

On 2.4 and higher, it is hard to predict consequences of this bug because
end of the message is only reported with a flag. But on 2.2 and lower, it
leads to a crash because there is an unexpected extra EOM block at the end
of an interim response.

Now, when a ES flag is detected on a HEADERS frame for an interim message, a
stream error is sent (RST_STREAM/PROTOCOL_ERROR).

This patch should solve the issue #1972. It should be backported as far as
2.0.

src/mux_h2.c

@@ -4782,6 +4782,11 @@ static int h2c_decode_headers(struct h2c *h2c, struct buffer *rxbuf, uint32_t *f
 		*flags |= H2_SF_HEADERS_RCVD;
 
 	if (h2c->dff & H2_F_HEADERS_END_STREAM) {
+		if (msgf & H2_MSGF_RSP_1XX) {
+			/* RFC9113#8.1 : HEADERS frame with the ES flag set that carries an informational status code is malformed */
+			TRACE_STATE("invalid interim response with ES flag!", H2_EV_RX_FRAME|H2_EV_RX_HDR|H2_EV_H2C_ERR|H2_EV_PROTO_ERR, h2c->conn);
+			goto fail;
+		}
 		/* no more data are expected for this message */
 		htx->flags |= HTX_FL_EOM;
 	}