smacker: validate number of channels

(cherry picked from commit e190e453bd)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/smacker.c

@@ -559,6 +559,10 @@ static av_cold int decode_end(AVCodecContext *avctx)
 
 static av_cold int smka_decode_init(AVCodecContext *avctx)
 {
+    if (avctx->channels < 1 || avctx->channels > 2) {
+        av_log(avctx, AV_LOG_ERROR, "invalid number of channels\n");
+        return AVERROR(EINVAL);
+    }
     avctx->channel_layout = (avctx->channels==2) ? AV_CH_LAYOUT_STEREO : AV_CH_LAYOUT_MONO;
     avctx->sample_fmt = avctx->bits_per_coded_sample == 8 ? AV_SAMPLE_FMT_U8 : AV_SAMPLE_FMT_S16;
     return 0;