Fix double free() error in TrackLoader class
Fix double free() error in TrackLoader class

This bug was introduced in commit 620092f45 due to changing progress bar from a C pointer to a Ptr<> class.

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