Fix `riegeli::tensorflow::FileReaderBase::ReadToBuffer()`, this time properly: if nothing new needs to be read, we need to simulate `src->Read()` reading nothing, and thus existing data in `buffer_` before `flat_buffer` must be preserved.
Fix `riegeli::tensorflow::FileReaderBase::ReadToBuffer()`, this time properly:
if nothing new needs to be read, we need to simulate `src->Read()` reading
nothing, and thus existing data in `buffer_` before `flat_buffer` must be
preserved.

PiperOrigin-RevId: 433170284

google/riegeli,google/riegeli,google/riegeli,google/riegeli