Fix crash in simple server when backend is missing

gfe-relnote: n/a, test-only change
PiperOrigin-RevId: 246231018
Change-Id: I804242d6e95229406d3f80dd0a657ea0928917e7

quic/tools/quic_simple_server_stream.cc

diff --git a/quic/tools/quic_simple_server_stream.cc b/quic/tools/quic_simple_server_stream.cc
index 4e25552..e3fc507 100644
--- a/quic/tools/quic_simple_server_stream.cc
+++ b/quic/tools/quic_simple_server_stream.cc
@@ -41,7 +41,9 @@
       quic_simple_server_backend_(quic_simple_server_backend) {}
 
 QuicSimpleServerStream::~QuicSimpleServerStream() {
-  quic_simple_server_backend_->CloseBackendResponseStream(this);
+  if (quic_simple_server_backend_) {
+    quic_simple_server_backend_->CloseBackendResponseStream(this);
+  }
 }
 
 void QuicSimpleServerStream::OnInitialHeadersComplete(
@@ -141,6 +143,12 @@
     return;
   }
 
+  if (quic_simple_server_backend_ == nullptr) {
+    QUIC_DVLOG(1) << "Backend is missing.";
+    SendErrorResponse();
+    return;
+  }
+
   // Fetch the response from the backend interface and wait for callback once
   // response is ready
   quic_simple_server_backend_->FetchResponseFromBackend(request_headers_, body_,