Adding defensive CHECKs for AudioDecoderConfig crash

Top frames of the crash stack don't make much sense. Checking the
obvious sources of memory access issues.

BUG=715761

Review-Url: https://codereview.chromium.org/2856633002
Cr-Commit-Position: refs/heads/master@{#468435}

media/filters/chunk_demuxer.cc

@@ -284,12 +284,16 @@ DemuxerStream::Liveness ChunkDemuxerStream::liveness() const {
 AudioDecoderConfig ChunkDemuxerStream::audio_decoder_config() {
   CHECK_EQ(type_, AUDIO);
   base::AutoLock auto_lock(lock_);
+  // Trying to track down crash. http://crbug.com/715761
+  CHECK(stream_);
   return stream_->GetCurrentAudioDecoderConfig();
 }
 
 VideoDecoderConfig ChunkDemuxerStream::video_decoder_config() {
   CHECK_EQ(type_, VIDEO);
   base::AutoLock auto_lock(lock_);
+  // Trying to track down crash. http://crbug.com/715761
+  CHECK(stream_);
   return stream_->GetCurrentVideoDecoderConfig();
 }
 

media/filters/source_buffer_stream.cc

@@ -1551,12 +1551,18 @@ bool SourceBufferStream::IsEndOfStreamReached() const {
 const AudioDecoderConfig& SourceBufferStream::GetCurrentAudioDecoderConfig() {
   if (config_change_pending_)
     CompleteConfigChange();
+  // Trying to track down crash. http://crbug.com/715761
+  CHECK(current_config_index_ >= 0 &&
+        static_cast<size_t>(current_config_index_) < audio_configs_.size());
   return audio_configs_[current_config_index_];
 }
 
 const VideoDecoderConfig& SourceBufferStream::GetCurrentVideoDecoderConfig() {
   if (config_change_pending_)
     CompleteConfigChange();
+  // Trying to track down crash. http://crbug.com/715761
+  CHECK(current_config_index_ >= 0 &&
+        static_cast<size_t>(current_config_index_) < video_configs_.size());
   return video_configs_[current_config_index_];
 }