Chrome OS: Fix crash in SigninScreenHandler::HandleAuthenticateUser.

SigninScreenHandler::HandleAuthenticateUser should ignore 'User not found'
errors.

Bug: 794487
Change-Id: I83c9a9bc2787514384e5bbee9253eaf383df94db
Reviewed-on: https://chromium-review.googlesource.com/850998Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527106}

chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc

@@ -1186,7 +1186,12 @@ void SigninScreenHandler::HandleAuthenticateUser(const AccountId& account_id,
   const user_manager::User* user =
       user_manager::UserManager::Get()->FindUser(account_id);
   DCHECK(user);
+  if (!user) {
+    LOG(ERROR) << "HandleAuthenticateUser: User not found! account type="
+               << AccountId::AccountTypeToString(account_id.GetAccountType());
+  } else {
     user_context.SetUserType(user->GetType());
+  }
   if (account_id.GetAccountType() == AccountType::ACTIVE_DIRECTORY)
     user_context.SetUserType(user_manager::USER_TYPE_ACTIVE_DIRECTORY);
   delegate_->Login(user_context, SigninSpecifics());