[Chromecast] Don't push buffers after the backend has been stopped

BUG= internal b/31488922

Review-Url: https://codereview.chromium.org/2340013002
Cr-Commit-Position: refs/heads/master@{#418636}

chromecast/media/cma/pipeline/av_pipeline_impl.cc

@@ -277,6 +277,10 @@ void AvPipelineImpl::OnBufferDecrypted(
     LOG(WARNING) << "Can't decrypt with decrypt_context";
     buffer->set_decrypt_context(std::move(decrypt_context));
   }
+
+  if (!enable_feeding_)
+    return;
+
   pending_buffer_ = buffer;
   PushPendingBuffer();
 }