avcodec/ac3dec: Fix null pointer dereference in ac3_decode_frame()

Fixes: index 8 out of bounds for type 'uint8_t *[8]'
Fixes: 7273/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EAC3_fuzzer-6296497667702784

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ac3dec.c

@@ -1738,7 +1738,9 @@ dependent_frame:
 
     for (ch = 0; ch < avctx->channels; ch++) {
         int map = extended_channel_map[ch];
-        memcpy((SHORTFLOAT *)frame->data[ch], s->output_buffer[map],
+        av_assert0(ch>=AV_NUM_DATA_POINTERS || frame->extended_data[ch] == frame->data[ch]);
+        memcpy((SHORTFLOAT *)frame->extended_data[ch],
+               s->output_buffer[map],
                s->num_blocks * AC3_BLOCK_SIZE * sizeof(SHORTFLOAT));
     }