Add some NULL-checks for ExtensionHost::extension to fix a crash bug.

chrome/browser/extensions/extension_event_router.cc

 // Copyright (c) 2012 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/extensions/extension_event_router.h"
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/message_loop.h"
 #include "base/values.h"
@@ skipping 413 matching lines @@
 }
 
 void ExtensionEventRouter::OnEventAck(
     Profile* profile, const std::string& extension_id) {
   ExtensionProcessManager* pm =
       ExtensionSystem::Get(profile)->process_manager();
   ExtensionHost* host = pm->GetBackgroundHostForExtension(extension_id);
   // The event ACK is routed to the background host, so this should never be
   // NULL.
   CHECK(host);
+  if (!host->extension())
+    return;
+
   CHECK(host->extension()->has_lazy_background_page());
   pm->DecrementLazyKeepaliveCount(host->extension());
 }
 
 void ExtensionEventRouter::DispatchPendingEvent(
     const linked_ptr<ExtensionEvent>& event, ExtensionHost* host) {
   if (!host)
     return;
 
   ListenerProcess listener(host->render_process_host(),
@@ skipping 60 matching lines @@
       MessageLoop::current()->PostTask(FROM_HERE,
           base::Bind(&extensions::RuntimeEventRouter::DispatchOnInstalledEvent,
                      profile_, extension->id()));
       break;
     }
     default:
       NOTREACHED();
       return;
   }
 }