Fix the crash that happens if the default wallpaper image decoding failed.

chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.cc

 // Copyright 2014 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/chromeos/login/users/wallpaper/wallpaper_manager.h"
 
 #include <utility>
 
 #include "ash/ash_constants.h"
 #include "ash/public/cpp/shelf_types.h"
@@ skipping 1348 matching lines @@
       chromeos::switches::kChildWallpaperLarge);
   default_wallpaper_image_.reset();
 }
 
 void WallpaperManager::OnDefaultWallpaperDecoded(
     const base::FilePath& path,
     const wallpaper::WallpaperLayout layout,
     std::unique_ptr<user_manager::UserImage>* result_out,
     MovableOnDestroyCallbackHolder on_finish,
     std::unique_ptr<user_manager::UserImage> user_image) {
+  if (user_image->image().isNull()) {
+    LOG(ERROR) << "Failed to decode default wallpaper. ";
+    return;
+  }
+
   *result_out = std::move(user_image);
   SetWallpaper((*result_out)->image(), layout);
 }
 
 void WallpaperManager::StartLoadAndSetDefaultWallpaper(
     const base::FilePath& path,
     const wallpaper::WallpaperLayout layout,
     MovableOnDestroyCallbackHolder on_finish,
     std::unique_ptr<user_manager::UserImage>* result_out) {
   user_image_loader::StartWithFilePath(
@@ skipping 52 matching lines @@
 
   UMA_HISTOGRAM_ENUMERATION(
       "Ash.Wallpaper.Apps",
       wallpaper_manager_util::ShouldUseAndroidWallpapersApp(profile)
           ? WALLPAPERS_APP_ANDROID
           : WALLPAPERS_PICKER_APP_CHROMEOS,
       WALLPAPERS_APPS_NUM);
 }
 
 }  // namespace chromeos