Fix crash when displaying a null certificate.

This regressed in r417444. Previously (https://codereview.chromium.org/2327433002/diff/250001/chrome/browser/certificate_viewer.cc) we would check that the certificate id sent by the renderer was valid (say because renderer died). Races could also happen now, in that the page might navigate right after user presses certificate viewer button and the last committed entry wouldn't have a certificate.

BUG=646882

Review-Url: https://codereview.chromium.org/2337783005
Cr-Commit-Position: refs/heads/master@{#418644}

content/browser/devtools/protocol/security_handler.cc

@@ -156,6 +156,8 @@ Response SecurityHandler::ShowCertificateViewer() {
   WebContents* web_contents = WebContents::FromRenderFrameHost(host_);
   scoped_refptr<net::X509Certificate> certificate = web_contents->
       GetController().GetLastCommittedEntry()->GetSSL().certificate;
+  if (!certificate)
+    return Response::InternalError("Could not find certificate");
   web_contents->GetDelegate()->ShowCertificateViewerInDevTools(
       web_contents, certificate);
   return Response::OK();