Fix potential NPE when saving fragment state.

If a fragment's saved view state is null and the user
visible hint is true then the `result` bundle will have
never been initialized to a value resulting in a
`NullPointerException`.

Mirrors support library change I8ba585bc6b9298841490d64bc22a8219cd261adb.

Change-Id: Iabd5ac293d2ece3771da9ef257479eca0dcd523c