revealed since...

commit 81e3ca4f
Author: Tor Lillqvist <tml@collabora.com>
Date:   Tue Jun 21 10:34:21 2016 +0300

surely here we should ask the palette how many entries are
in it, not assume from the original bitcount that all entries
exist.

presumably the palette can only have <= 1 << BitCount entries in it

Change-Id: Ieb1b98f2f13f702a6a6a20d8cf3d8e9a695141b2