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

Author: JakeWharton

core/java/android/app/FragmentManager.java

@@ -1538,6 +1538,9 @@ Bundle saveFragmentBasicState(Fragment f) {
                     FragmentManagerImpl.VIEW_STATE_TAG, f.mSavedViewState);
         }
         if (!f.mUserVisibleHint) {
+            if (result == null) {
+                result = new Bundle();
+            }
             // Only add this if it's not the default value
             result.putBoolean(FragmentManagerImpl.USER_VISIBLE_HINT_TAG, f.mUserVisibleHint);
         }