avformat/adxdec: set avctx->channels in adx_read_header

It is used in adx_read_packet, which currently depends on the decoder/parser setting this value between reading the file header and demuxing the first packet.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/adxdec.c

@@ -83,8 +83,14 @@ static int adx_read_header(AVFormatContext *s)
         av_log(s, AV_LOG_ERROR, "Invalid extradata size.\n");
         return AVERROR_INVALIDDATA;
     }
+    avctx->channels    = AV_RB8(avctx->extradata + 7);
     avctx->sample_rate = AV_RB32(avctx->extradata + 8);
 
+    if (avctx->channels <= 0) {
+        av_log(s, AV_LOG_ERROR, "invalid number of channels %d\n", avctx->channels);
+        return AVERROR_INVALIDDATA;
+    }
+
     st->codec->codec_type  = AVMEDIA_TYPE_AUDIO;
     st->codec->codec_id    = s->iformat->raw_codec_id;