BUG: Some minor cleanup and improvement after itkNiftiImageIO code review
BUG: Some minor cleanup and improvement after itkNiftiImageIO code review

- fixed leak, added free() to balance malloc()
- don't guard nifti_image_free() will null check, it does that internally
- added some consts
- reduce copies by making some parameters references
- changed some variable types to match their source and avoid theoretical truncation

Change-Id: I8a0ec3fd193affb3be1c920c49f5ccb7c5358e30

LucasGandel/ITK,richardbeare/ITK,thewtex/ITK,vfonov/ITK,vfonov/ITK,fbudin69500/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,vfonov/ITK,hjmjohnson/ITK,BRAINSia/ITK,stnava/ITK,LucasGandel/ITK,BRAINSia/ITK,fbudin69500/ITK,vfonov/ITK,Kitware/ITK,LucasGandel/ITK,LucasGandel/ITK,richardbeare/ITK,malaterre/ITK,richardbeare/ITK,richardbeare/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,blowekamp/ITK,malaterre/ITK,spinicist/ITK,stnava/ITK,stnava/ITK,fbudin69500/ITK,spinicist/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,fbudin69500/ITK,Kitware/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,vfonov/ITK,spinicist/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,blowekamp/ITK,blowekamp/ITK,InsightSoftwareConsortium/ITK,stnava/ITK,stnava/ITK,hjmjohnson/ITK,fbudin69500/ITK,malaterre/ITK,LucasGandel/ITK,spinicist/ITK,stnava/ITK,LucasGandel/ITK,vfonov/ITK,stnava/ITK,LucasGandel/ITK,fbudin69500/ITK,BRAINSia/ITK,vfonov/ITK,BRAINSia/ITK,Kitware/ITK,spinicist/ITK,Kitware/ITK,spinicist/ITK,BRAINSia/ITK,fbudin69500/ITK,blowekamp/ITK,spinicist/ITK,InsightSoftwareConsortium/ITK,spinicist/ITK,malaterre/ITK,hjmjohnson/ITK,thewtex/ITK,malaterre/ITK,BRAINSia/ITK,malaterre/ITK,hjmjohnson/ITK,hjmjohnson/ITK,blowekamp/ITK,blowekamp/ITK,blowekamp/ITK,Kitware/ITK,richardbeare/ITK,thewtex/ITK,stnava/ITK,thewtex/ITK,vfonov/ITK,blowekamp/ITK,stnava/ITK,spinicist/ITK,Kitware/ITK,thewtex/ITK,hjmjohnson/ITK,vfonov/ITK,richardbeare/ITK,thewtex/ITK,fbudin69500/ITK,malaterre/ITK