[Chromecast] Fix potential nullptr dereference.

Guard call to SetStreamTypeVolume in MediaPipelineBackendWrapper
with a check for audio_decoder_wrapper_. An audio decoder wrapper
is not guarenteed to be initialized.

Also adds DCHECKs to enforce correct API usage of Create*Decoder
and Initialize.

R=halliwell@chromium.org
BUG=internal b/27596803
TEST=Play video content with no audio track

Review URL: https://codereview.chromium.org/1786563002

Cr-Commit-Position: refs/heads/master@{#380522}

1 files changed, 4 insertions, 1 deletions

diff --git a/chromecast/media/cma/backend/media_pipeline_backend_wrapper.cc b/chromecast/media/cma/backend/media_pipeline_backend_wrapper.cc
index 447981f..ef86099 100644
--- a/chromecast/media/cma/backend/media_pipeline_backend_wrapper.cc
+++ b/chromecast/media/cma/backend/media_pipeline_backend_wrapper.cc
@@ -28,6 +28,7 @@ MediaPipelineBackendWrapper::~MediaPipelineBackendWrapper() {
 
 MediaPipelineBackend::AudioDecoder*
 MediaPipelineBackendWrapper::CreateAudioDecoder() {
+  DCHECK(!is_initialized_);
   if (audio_decoder_wrapper_)
     return nullptr;
 
@@ -38,12 +39,14 @@ MediaPipelineBackendWrapper::CreateAudioDecoder() {
 
 MediaPipelineBackend::VideoDecoder*
 MediaPipelineBackendWrapper::CreateVideoDecoder() {
+  DCHECK(!is_initialized_);
   return backend_->CreateVideoDecoder();
 }
 
 bool MediaPipelineBackendWrapper::Initialize() {
+  DCHECK(!is_initialized_);
   is_initialized_ = backend_->Initialize();
-  if (is_initialized_)
+  if (is_initialized_ && audio_decoder_wrapper_)
     audio_decoder_wrapper_->SetStreamTypeVolume(stream_type_volume_);
 
   return is_initialized_;