xxan: Disallow odd width

Decoded data is always written in pairs within this decoder.
This fixes writes out of bounds.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit aa0dd52434768da64f1f3d8ae92bcf980c1adffc)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>