MOQT: Check for nullptr in GetStreamById().

visitor()->session() is deliberately nullptr when closing.

PiperOrigin-RevId: 998979542

quiche/quic/moqt/moqt_live_publisher.h

diff --git a/quiche/quic/moqt/moqt_live_publisher.h b/quiche/quic/moqt/moqt_live_publisher.h
index 757cd9a..3f859e4 100644
--- a/quiche/quic/moqt/moqt_live_publisher.h
+++ b/quiche/quic/moqt/moqt_live_publisher.h
@@ -242,7 +242,7 @@
   }
 
   webtransport::Stream* GetStreamById(webtransport::StreamId stream_id) {
-    if (visitor() != nullptr) {
+    if (visitor() != nullptr && visitor()->session() != nullptr) {
       return visitor()->session()->GetStreamById(stream_id);
     }
     return nullptr;