The first 32 elements of each row were correct, while the
last 16 were scrambled.

This hasn't been noticed, because the checkasm test erroneously
only checked half of the output (for 8 bit functions), and
apparently none of the samples as part of "fate-hevc" seem to
trigger this specific function.
Signed-off-by: J. Dekker <jdek@itanimul.li>