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: I4f1a09dcb7d6a115d589d17dd28bdc94f0f00597
Signed-off-by: Roman Birg <roman@cyngn.com>