MLBEDSW-6626: Initialize lut_val in mlw_codec

The palette variable located on the stack was not properly
initialized and could potentially overwrite the stack memory
when palette size was increased to 2.
Make sure lut value is initialized.

Signed-off-by: Johan Alfven <johan.alfven@arm.com>
Change-Id: I9fecfe218dc39c0157d1af015e725d1e4becf2f0