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

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

Index: chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.cc
diff --git a/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.cc b/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.cc
index db81b6b383fd8110cabb1d83fa4b3b9af33b8f00..4738e13a09ce832003f80acb97965b6a5669a8d6 100644
--- a/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.cc
+++ b/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.cc
@@ -1366,6 +1366,11 @@ void WallpaperManager::OnDefaultWallpaperDecoded(
     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);
 }