Fixed a bug when returning the minimum value for voxels of type unsigned char
Fixed a bug when returning the minimum value for voxels of type unsigned char

ghisvail/irtk-legacy,BioMedIA/IRTK,BioMedIA/irtk-legacy,BioMedIA/irtk-legacy,sk1712/IRTK,sk1712/IRTK,BioMedIA/IRTK,ghisvail/irtk-legacy,ghisvail/irtk-legacy,sk1712/IRTK,BioMedIA/irtk-legacy,sk1712/IRTK,BioMedIA/IRTK,ghisvail/irtk-legacy,BioMedIA/IRTK,BioMedIA/irtk-legacy