vqavideo: return error if image size is not a multiple of block size

The decoder assumes in various places that the image size
is a multiple of the block size, and there is no obvious
way to support odd sizes.  Bailing out early if the header
specifies a bad size avoids various errors later on.

Fixes CVE-2012-0947.

Signed-off-by: Mans Rullgard <mans@mansr.com>
