Fix deprecation warning from new ffmpeg (#416)

Closes #415

Signed-off-by: Michael Carroll <[email protected]>

Author: mjcarroll

av/src/AudioDecoder.cc

@@ -149,7 +149,7 @@ bool AudioDecoder::Decode(uint8_t **_outBuffer, unsigned int *_outBufferSize)
         // decodedFrame->linesize[0].
         int size = decodedFrame->nb_samples *
           av_get_bytes_per_sample(this->dataPtr->codecCtx->sample_fmt) *
-          this->dataPtr->codecCtx->channels;
+          this->dataPtr->codecCtx->ch_layout.nb_channels;
 
         // Resize the audio buffer as necessary
         if (*_outBufferSize + size > maxBufferSize)