Improve test coverage with a "correctly" damaged palette index test

The PNG IDAT did not include a '255' entry, the highest entry is '254',
this corrects the test PNG to have a palette with only 254 entries so
that it triggers the palette index checks.

Signed-off-by: John Bowler <[email protected]>
Signed-off-by: Cosmin Truta <[email protected]>

Author: jbowler