message: Add back setting of the null feed buffers
  Summary: This got removed when I added page alignment.
  Reviewers: #tensorflow, davidn
diff: @@ -495,6 +495,7 @@ class NullPrefetchCallback : public poplar::StreamCallback {
  : num_bytes_(num_bytes), allocator_(allocator) {
  for (auto& buffer : buffers_) {
  buffer = static_cast<uint8*>(allocator_->AllocateRaw(64, num_bytes));
  + std::memset(buffer, 0x2, num_bytes);
  }
  }
  