avcodec: va: assert sooner when the data are not right
avcodec: va: assert sooner when the data are not right

Signed-off-by: Rémi Denis-Courmont <f0699137713be92736351a0674e79536595e4a60@remlab.net>

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc