WallpaperCropper: do not display null bitmaps

It is possible that themes can include a corrupted bitmap,
and the lockscreen will try and display a NULL image resulting in an
empty thumbnail and applying it will cause the app to crash.

Change-Id: Ie0770486a9d58050a5f296a219d844a39937421e
Signed-off-by: Roman Birg <roman@cyngn.com>