Commit 900ce6f8 replaced
IntraX8Context.ac_vlc by IntraX8Context.ac_vlc_table,
but forgot to update an av_assert2()*.
cf7ed019 then
replaced this with a check for j_ac_vlc[mode],
but this makes no sense as j_ac_vlc is of type
const VLCElem [2][2][8][].

Worse yet, mode can be up to three and then
j_ac_vlc[mode] is undefined behaviour. This happened
during the wmv8-x8intra FATE test.

*: Since 84f16bb5
config.h was no longer auto-included in avassert.h
and this disabled av_assert1() and av_assert2()
in files where config.h has not been included before
the inclusion of avassert.h.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>