avcodec/intrax8: Fix assert

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>

libavcodec/intrax8.c

@@ -103,7 +103,7 @@ static inline void x8_select_ac_table(IntraX8Context *const w, int mode)
     table_index       = get_bits(w->gb, 3);
     // 2 modes use same tables
     w->j_ac_vlc_table[mode] = j_ac_vlc[w->quant < 13][mode >> 1][table_index];
-    av_assert2(j_ac_vlc[mode]);
+    av_assert2(w->j_ac_vlc_table[mode]);
 }
 
 static inline int x8_get_orient_vlc(IntraX8Context *w)