Fix url loader when interceptor allows redirect to continue.

R=ppi@chromium.org

Review URL: https://codereview.chromium.org/1209573002.

mojo/services/network/url_loader_impl.cc

diff --git a/mojo/services/network/url_loader_impl.cc b/mojo/services/network/url_loader_impl.cc
index 1f63ccc..10e497a 100644
--- a/mojo/services/network/url_loader_impl.cc
+++ b/mojo/services/network/url_loader_impl.cc
@@ -411,7 +411,8 @@
 
 void URLLoaderImpl::OnIntercept(
     URLLoaderInterceptorResponsePtr interceptor_response) {
-  if (interceptor_response->request && interceptor_response->response) {
+  if (interceptor_response && interceptor_response->request &&
+      interceptor_response->response) {
     SendError(net::ERR_INVALID_ARGUMENT, callback_);
     return;
   }