Check for nullptr in TlsServerHandshaker

Add nullptr check, not flag protected

PiperOrigin-RevId: 310426354
Change-Id: I7a6d88377a1a551a66f3f9ce51fea682c30b4b0a
diff --git a/quic/core/tls_server_handshaker.cc b/quic/core/tls_server_handshaker.cc
index 229e568..c8fbda5 100644
--- a/quic/core/tls_server_handshaker.cc
+++ b/quic/core/tls_server_handshaker.cc
@@ -502,7 +502,7 @@
       proof_source_->GetCertChain(session()->connection()->self_address(),
                                   session()->connection()->peer_address(),
                                   hostname_);
-  if (chain->certs.empty()) {
+  if (!chain || chain->certs.empty()) {
     QUIC_LOG(ERROR) << "No certs provided for host '" << hostname_ << "'";
     return SSL_TLSEXT_ERR_ALERT_FATAL;
   }