Fix to Tractography::WriterUnbuffered class
Fix to Tractography::WriterUnbuffered class

A bug was introduced during development of the track_writer branch which resulted in the unbuffered writer not filling the output file with streamlines data.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3