sct_get_centerline: fixup mistakes in image refactor (#2002)
sct_get_centerline: fixup mistakes in image refactor (#2002)


Former-commit-id: 02cd22e20c7876a11705de6a10ca382e3330a7ed
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox