Fix -Wstringop-truncation warnings in vtkNIFTIImageWriter.cxx
These turned up on vall
(ParaView dashboard): https://open.cdash.org/viewBuildError.php?type=1&buildid=5458931
These turned up on vall
(ParaView dashboard): https://open.cdash.org/viewBuildError.php?type=1&buildid=5458931