DoH: reduce excessive bad request logging

We started using isc_nm_bad_request() more actively throughout
codebase. In the case of HTTP/2 it can lead to a large count of
useless "Bad Request" messages in the BIND log, as often we attempt to
send such request over effectively finished HTTP/2 sessions.

This commit fixes that.

lib/isc/netmgr/http.c

@@ -2484,6 +2484,12 @@ isc__nm_http_bad_request(isc_nmhandle_t *handle) {
 	REQUIRE(!sock->client);
 	REQUIRE(VALID_HTTP2_SESSION(sock->h2->session));
 
+	if (sock->h2->response_submitted ||
+	    !http_session_active(sock->h2->session))
+	{
+		return;
+	}
+
 	(void)server_send_error_response(ISC_HTTP_ERROR_BAD_REQUEST,
 					 sock->h2->session->ngsession, sock);
 }