cros: Attempt to fix crash when suspending on login

CurrentBigUserView() could be null if
- there are no users on the device yet
- enterprise policy disables showing the logged in users

Bug: 889580
Change-Id: If7eea97b84697dfc2ced137741489a41f7520993
Reviewed-on: https://chromium-review.googlesource.com/1249815Reviewed-by: Quan Nguyen <qnnguyen@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594860}

ash/login/ui/lock_contents_view.cc

@@ -1009,9 +1009,9 @@ void LockContentsView::OnStateChanged(
 
 void LockContentsView::SuspendImminent(
     power_manager::SuspendImminent::Reason reason) {
-  LoginAuthUserView* auth_user = CurrentBigUserView()->auth_user();
-  if (auth_user)
-    auth_user->password_view()->Clear();
+  LoginBigUserView* big_user = CurrentBigUserView();
+  if (big_user && big_user->auth_user())
+    big_user->auth_user()->password_view()->Clear();
 }
 
 void LockContentsView::ShowAuthErrorMessageForDebug(int unlock_attempt) {