OPT: correct bug in sct_segment_graymatter when -denoising 0
OPT: correct bug in sct_segment_graymatter when -denoising 0

Former-commit-id: 8963ef366c186d291bb265cc2dfe1eee6cbbc90e
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox