Comment stream cleanup more to resolve nits

Author: pimterry

src/quic/application.cc

@@ -210,6 +210,7 @@ Session::Application::ExtractSessionTicketAppData(
 void Session::Application::ReceiveStreamClose(stream_id id,
                                               Stream* stream,
                                               QuicError&& error) {
+  // Stream may be nullptr if our side is already gone
   if (stream == nullptr) return;
   stream->Destroy(std::move(error));
 }

src/quic/session.cc

@@ -1582,7 +1582,11 @@ struct Session::Impl final : public MemoryRetainer {
     // If the peer closes a stream, we return the credit to allow a new one:
     session->ExtendMaxStreams(stream_id);
     if (!session->has_application()) return NGTCP2_SUCCESS;
+
+    // Stream here may be null (e.g. if already destroyed locally) so we also
+    // pass stream_id to the calls below as well.
     auto* stream = Stream::From(stream_user_data);
+
     if (flags & NGTCP2_STREAM_CLOSE_FLAG_APP_ERROR_CODE_SET) {
       session->application().ReceiveStreamClose(
           stream_id, stream, QuicError::ForApplication(app_error_code));