update channel_layout by channels, for ffmpeg transcode opus to aac success (#2452)

trunk/src/app/srs_app_rtc_codec.cpp

@@ -148,6 +148,8 @@ srs_error_t SrsAudioTranscoder::init_dec(SrsAudioCodecId src_codec)
         return srs_error_new(ERROR_RTC_RTP_MUXER, "Could not open codec");
     }
     
+    dec_->channel_layout = av_get_default_channel_layout(dec_->channels);
+
     dec_frame_ = av_frame_alloc();
     if (!dec_frame_) {
         return srs_error_new(ERROR_RTC_RTP_MUXER, "Could not allocate audio decode out frame");