Fix two crash and potential leak&crash

ash/accelerators/accelerator_dispatcher_win.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 "ash/accelerators/accelerator_dispatcher.h"
 
 #include "ash/accelerators/accelerator_controller.h"
 #include "ash/shell.h"
 #include "ui/aura/event.h"
 #include "ui/aura/root_window.h"
 #include "ui/base/accelerators/accelerator.h"
 
 namespace ash {
 
 namespace {
 
 const int kModifierMask = (ui::EF_SHIFT_DOWN |
                            ui::EF_CONTROL_DOWN |
                            ui::EF_ALT_DOWN);
 }  // namespace
 
 bool AcceleratorDispatcher::Dispatch(const MSG& msg) {
+  if (!associated_window_)
+    return false;
   if (!associated_window_->CanReceiveEvents())
     return aura::RootWindow::GetInstance()->GetDispatcher()->Dispatch(msg);
 
   if(msg.message == WM_KEYDOWN || msg.message == WM_SYSKEYDOWN) {
     ash::AcceleratorController* accelerator_controller =
         ash::Shell::GetInstance()->accelerator_controller();
     ui::Accelerator accelerator(ui::KeyboardCodeFromNative(msg),
         ui::EventFlagsFromNative(msg) & kModifierMask);
     if (accelerator_controller && accelerator_controller->Process(accelerator))
       return true;
   }
 
   return nested_dispatcher_->Dispatch(msg);
 }
 
 }  // namespace ash