Fix `riegeli::tensorflow::FileReaderBase::ReadToBuffer()`: `buffer_` and buffer pointers must be updated also in case of an early return.
Fix `riegeli::tensorflow::FileReaderBase::ReadToBuffer()`: `buffer_` and buffer
pointers must be updated also in case of an early return.

PiperOrigin-RevId: 433007339

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