[Chromecast] Fix uninitialized data in ALSA mixer unittest

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chromecast/media/cma/backend/alsa/stream_mixer_alsa.h"
 
 #include <algorithm>
 #include <cmath>
 #include <limits>
 #include <utility>
@@ skipping 161 matching lines @@
   const ::media::AudioBus& data() {
     CHECK(data_);
     return *data_;
   }
   float multiplier() const { return multiplier_; }
 
  private:
   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) {
     deleting_ = true;
     delete_cb.Run(this);
   }
 
   bool paused_;
   int samples_per_second_;
   int max_read_size_;
@@ skipping 395 matching lines @@
 
   input->SetMaxReadSize(1024);
   EXPECT_CALL(*input, GetResampledData(_, 1024));
   EXPECT_CALL(*input, AfterWriteFrames(_));
   EXPECT_CALL(*mock_alsa(), PcmWritei(_, _, 1024)).Times(1);
   mixer->WriteFramesForTest();
 }
 
 }  // namespace media
 }  // namespace chromecast