avcodec/atrac3plus: Check split point in fill mode 3

Fixes: index 32 out of bounds for type 'int [32]'
Fixes: 18350/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ATRAC3P_fuzzer-5643794862571520

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 de5102fd92)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/atrac3plus.c

@@ -459,6 +459,10 @@ static int decode_channel_wordlen(GetBitContext *gb, Atrac3pChanUnitCtx *ctx,
     } else if (chan->fill_mode == 3) {
         pos = ch_num ? chan->num_coded_vals + chan->split_point
                      : ctx->num_quant_units - chan->split_point;
+        if (pos > FF_ARRAY_ELEMS(chan->qu_wordlen)) {
+            av_log(avctx, AV_LOG_ERROR, "Split point beyond array\n");
+            pos = FF_ARRAY_ELEMS(chan->qu_wordlen);
+        }
         for (i = chan->num_coded_vals; i < pos; i++)
             chan->qu_wordlen[i] = 1;
     }