Image::ThreadedLoop: fix silly bug that caused race condition at loop start
Image::ThreadedLoop: fix silly bug that caused race condition at loop start

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