fixelcfestats: Fix segfault when using input tractogram and -mask option
fixelcfestats: Fix segfault when using input tractogram and -mask option

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