avcodec/dnxhddec: check and propagate function return value

Similar to CVE-2013-0868, here return value check for 'init_vlc' is needed.
crafted DNxHD data can cause unspecified impact.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>