Keyguard: fix NPE setting lockscreen wallpaper

After boot the view might not be laid out and so getWidth() and
getHeight() return 0 and will cause a NullPointerException in
createBitmap().

Change-Id: Ibacf4199a69709dc34d21695732c2a5360533a23
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>