mpc7: only support stereo input.

The Musepack SV7 reference encoder converts mono to stereo when encoding.

libavcodec/mpc7.c

@@ -61,6 +61,13 @@ static av_cold int mpc7_decode_init(AVCodecContext * avctx)
     static VLC_TYPE hdr_table[1 << MPC7_HDR_BITS][2];
     static VLC_TYPE quant_tables[7224][2];
 
+    /* Musepack SV7 is always stereo */
+    if (avctx->channels != 2) {
+        av_log_ask_for_sample(avctx, "Unsupported number of channels: %d\n",
+                              avctx->channels);
+        return AVERROR_PATCHWELCOME;
+    }
+
     if(avctx->extradata_size < 16){
         av_log(avctx, AV_LOG_ERROR, "Too small extradata size (%i)!\n", avctx->extradata_size);
         return -1;
@@ -88,7 +95,7 @@ static av_cold int mpc7_decode_init(AVCodecContext * avctx)
     c->frames_to_skip = 0;
 
     avctx->sample_fmt = AV_SAMPLE_FMT_S16;
-    avctx->channel_layout = (avctx->channels==2) ? AV_CH_LAYOUT_STEREO : AV_CH_LAYOUT_MONO;
+    avctx->channel_layout = AV_CH_LAYOUT_STEREO;
 
     if(vlc_initialized) return 0;
     av_log(avctx, AV_LOG_DEBUG, "Initing VLC\n");