avcodec/aptx: Check the number of channels

Fixes: store to null pointer of type 'uint32_t' (aka 'unsigned int')
Fixes: 18021/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APTX_HD_fuzzer-5761738313564160

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 98a257c323)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/aptx.c

@@ -989,6 +989,9 @@ static av_cold int aptx_init(AVCodecContext *avctx)
     AptXContext *s = avctx->priv_data;
     int chan, subband;
 
+    if (avctx->channels != 2)
+        return AVERROR_INVALIDDATA;
+
     s->hd = avctx->codec->id == AV_CODEC_ID_APTX_HD;
     s->block_size = s->hd ? 6 : 4;