Don't crash if DontProceed is chosen on the SSL interstitial page after Proceed is chosen.

chrome/browser/ssl/ssl_blocking_page.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ssl/ssl_blocking_page.h"
 
 #include "base/i18n/rtl.h"
 #include "base/metrics/histogram.h"
 #include "base/string_piece.h"
 #include "base/utf_string_conversions.h"
@@ skipping 128 matching lines @@
 }
 
 void SSLBlockingPage::DontProceed() {
   RecordSSLBlockingPageStats(DONT_PROCEED);
 
   NotifyDenyCertificate();
   InterstitialPage::DontProceed();
 }
 
 void SSLBlockingPage::NotifyDenyCertificate() {
-  DCHECK(callback_);
+  // It's possible that callback_ may not exist if the user clicks "Proceed"
+  // followed by pressing the back button before the interstitial is hidden.
+  // In that case the certificate will still be treated as allowed.
+  if (!callback_)
+    return;
 
   callback_->Run(handler_, false);
   delete callback_;
   callback_ = NULL;
 }
 
 void SSLBlockingPage::NotifyAllowCertificate() {
   DCHECK(callback_);

[wtc, 2011/09/02 20:43:15]

Does this DCHECK need the same treatment?

 
   callback_->Run(handler_, true);
   delete callback_;
   callback_ = NULL;
 }
 
 // static
 void SSLBlockingPage::SetExtraInfo(
     DictionaryValue* strings,
     const std::vector<string16>& extra_info) {
   DCHECK(extra_info.size() < 5);  // We allow 5 paragraphs max.
   const char* keys[5] = {
       "moreInfo1", "moreInfo2", "moreInfo3", "moreInfo4", "moreInfo5"
   };
   int i;
   for (i = 0; i < static_cast<int>(extra_info.size()); i++) {
     strings->SetString(keys[i], extra_info[i]);
   }
   for (; i < 5; i++) {
     strings->SetString(keys[i], "");
   }
 }