avcodec/cbs_jpeg_syntax_template: Check table index before use in dht()

Fixes: out of array access
Fixes: 21515/clusterfuzz-testcase-minimized-ffmpeg_BSF_TRACE_HEADERS_fuzzer-5766121576988672

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit d31862c2b1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cbs_jpeg_syntax_template.c

@@ -108,6 +108,9 @@ static int FUNC(dht)(CodedBitstreamContext *ctx, RWContext *rw,
 
     n = 2;
     for (i = 0; n < current->Lh; i++) {
+        if (i >= 8)
+            return AVERROR_INVALIDDATA;
+
         CHECK(FUNC(huffman_table)(ctx, rw, &current->table[i]));
 
         ++n;