Fix open-source build

cl/477725360 broke the open-source build, for example in Envoy

external/com_github_google_quiche/quiche/quic/core/quic_dispatcher.cc:1111:23: error: use of undeclared identifier 'session'; did you mean 'session2'?
      if (session2 == session || cid == server_connection_id) {

PiperOrigin-RevId: 478109222
diff --git a/quiche/quic/core/quic_dispatcher.cc b/quiche/quic/core/quic_dispatcher.cc
index acce420..63fa567 100644
--- a/quiche/quic/core/quic_dispatcher.cc
+++ b/quiche/quic/core/quic_dispatcher.cc
@@ -1090,6 +1090,7 @@
       << ") due to error: " << QuicErrorCodeToString(error)
       << ", with details: " << error_details;
 
+  const QuicSession* session = it->second.get();
   QuicConnection* connection = it->second->connection();
   // Set up alarm to fire immediately to bring destruction of this session
   // out of current call stack.