cros: For arrow-nav in views login, properly handle "no users" and "no user focused" cases.

Previously, the login screen would crash in both these cases (e.g. if
the powerwash confirmation dialog popped up and the user pressed
left/right arrow).

Change-Id: I5880d72112508de93e77c507239ea28c475d86ae
Reviewed-on: https://chromium-review.googlesource.com/1174907
Commit-Queue: Quan Nguyen <qnnguyen@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583083}

ash/login/ui/lock_contents_view.cc

@@ -435,6 +435,10 @@ void LockContentsView::FocusNextUser() {
     return;
   }
 
+  if (!users_list_) {
+    return;
+  }
+
   for (int i = 0; i < users_list_->user_count(); ++i) {
     LoginUserView* user_view = users_list_->user_view_at(i);
     if (!login_views_utils::HasFocusInAnyChildView(user_view))
@@ -469,6 +473,10 @@ void LockContentsView::FocusPreviousUser() {
     return;
   }
 
+  if (!users_list_) {
+    return;
+  }
+
   for (int i = 0; i < users_list_->user_count(); ++i) {
     LoginUserView* user_view = users_list_->user_view_at(i);
     if (!login_views_utils::HasFocusInAnyChildView(user_view))