[Chromecast] Fix uninitialized data in ALSA mixer unittest

chromecast/media/cma/backend/alsa/stream_mixer_alsa_unittest.cc

Index: chromecast/media/cma/backend/alsa/stream_mixer_alsa_unittest.cc
diff --git a/chromecast/media/cma/backend/alsa/stream_mixer_alsa_unittest.cc b/chromecast/media/cma/backend/alsa/stream_mixer_alsa_unittest.cc
index dbe841bd30e83704621a34a7ce9b9920679187f7..9017e1506b693fbf224a83aaa9837bb0ccf6c795 100644
--- a/chromecast/media/cma/backend/alsa/stream_mixer_alsa_unittest.cc
+++ b/chromecast/media/cma/backend/alsa/stream_mixer_alsa_unittest.cc
@@ -179,8 +179,11 @@ class MockInputQueue : public StreamMixerAlsa::InputQueue {
   void DoGetResampledData(::media::AudioBus* dest, int frames) {
     CHECK(dest);
     CHECK_GE(dest->frames(), frames);
-    if (data_)
+    if (data_) {
       data_->CopyPartialFramesTo(0, frames, 0, dest);
+    } else {
+      dest->ZeroFramesPartial(0, frames);
+    }
   }
 
   void DoPrepareToDelete(const OnReadyToDeleteCb& delete_cb) {