audio: add function to fill frames with silence

When mixing audio frames, the mix buffer needs to be initialized with
silence.

Change-Id: I15d0573d8036a2755cbc33bdf91619cfedcf8f2d

src/client/videomanager.cpp

@@ -35,6 +35,7 @@
 #include "client/ring_signal.h"
 #include "audio/ringbufferpool.h"
 #include "dring/media_const.h"
+#include "libav_utils.h"
 
 #include <functional>
 #include <memory>
@@ -108,6 +109,7 @@ AudioFrame::mix(const AudioFrame& frame)
     }
     if (f.nb_samples == 0) {
         reserve(fIn.nb_samples);
+        ring::libav_utils::fillWithSilence(&f);
     } else if (f.nb_samples != fIn.nb_samples) {
         throw std::invalid_argument("Can't mix frames with different length");
     }

src/media/libav_deps.h

@@ -38,6 +38,7 @@ extern "C" {
 #include <libavutil/imgutils.h>
 #include <libavutil/intreadwrite.h>
 #include <libavutil/log.h>
+#include <libavutil/samplefmt.h>
 }
 
 #include "libav_utils.h"

src/media/libav_utils.cpp

@@ -222,4 +222,12 @@ fillWithBlack(AVFrame* frame)
     }
 }
 
+void
+fillWithSilence(AVFrame* frame)
+{
+    int ret = av_samples_set_silence(frame->extended_data, 0, frame->nb_samples, frame->channels, (AVSampleFormat)frame->format);
+    if (ret < 0)
+        RING_ERR() << "Failed to fill frame with silence";
+}
+
 }} // namespace ring::libav_utils

src/media/libav_utils.h

@@ -47,4 +47,6 @@ namespace ring { namespace libav_utils {
 
     void fillWithBlack(AVFrame* frame);
 
+    void fillWithSilence(AVFrame* frame);
+
 }} // namespace ring::libav_utils