mpc8: fix channel checks

fix heap array overflow
Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpc8.c

@@ -140,7 +140,8 @@ static av_cold int mpc8_decode_init(AVCodecContext * avctx)
     c->frames = 1 << (get_bits(&gb, 3) * 2);
 
     avctx->sample_fmt = AV_SAMPLE_FMT_S16;
-    avctx->channel_layout = (avctx->channels==2) ? AV_CH_LAYOUT_STEREO : AV_CH_LAYOUT_MONO;
+    avctx->channel_layout = (channels==2) ? AV_CH_LAYOUT_STEREO : AV_CH_LAYOUT_MONO;
+    avctx->channels = channels;
 
     if(vlc_initialized) return 0;
     av_log(avctx, AV_LOG_DEBUG, "Initing VLC\n");