Add check for changing number of channels in DCA.

Fixes issue 2505.

libavcodec/dca.c

@@ -273,6 +273,7 @@ typedef struct {
 
     /* Primary audio coding header */
     int subframes;              ///< number of subframes
+    int is_channels_set;        ///< check for if the channel number is already set
     int total_channels;         ///< number of channels including extensions
     int prim_channels;          ///< number of primary audio channels
     int subband_activity[DCA_PRIM_CHANNELS_MAX];    ///< subband activity count
@@ -1774,7 +1775,15 @@ static int dca_decode_frame(AVCodecContext * avctx,
        unset. Ideally during the first probe for channels the crc should be checked
        and only set avctx->channels when the crc is ok. Right now the decoder could
        set the channels based on a broken first frame.*/
+    if (s->is_channels_set == 0) {
+        s->is_channels_set = 1;
     avctx->channels = channels;
+    }
+    if (avctx->channels != channels) {
+        av_log(avctx, AV_LOG_ERROR, "DCA decoder does not support number of "
+               "channels changing in stream. Skipping frame.\n");
+        return -1;
+    }
 
     if (*data_size < (s->sample_blocks / 8) * 256 * sizeof(int16_t) * channels)
         return -1;