Fix strange use of session_detect_idle_stream

lib/nghttp2_session.c

@@ -3429,6 +3429,8 @@ int nghttp2_session_on_request_headers_received(nghttp2_session *session,
     return NGHTTP2_ERR_IGN_HEADER_BLOCK;
   }
 
+  assert(session->server);
+
   if (!session_is_new_peer_stream_id(session, frame->hd.stream_id)) {
     /* The spec says if an endpoint receives a HEADERS with invalid
        stream ID, it MUST issue connection error with error code
@@ -3437,7 +3439,8 @@ int nghttp2_session_on_request_headers_received(nghttp2_session *session,
        Then connection error is too harsh.  It means that we only use
        connection error if stream ID refers idle stream.  OTherwise we
        just ignore HEADERS for now. */
-    if (session_detect_idle_stream(session, frame->hd.stream_id)) {
+    if (frame->hd.stream_id == 0 ||
+        nghttp2_session_is_my_stream_id(session, frame->hd.stream_id)) {
       return session_inflate_handle_invalid_connection(
           session, frame, NGHTTP2_ERR_PROTO,
           "request HEADERS: invalid stream_id");