BUG: keep same volume when cropping with an uncomplete segmentation. Fix #879
BUG: keep same volume when cropping with an uncomplete segmentation. Fix #879

Former-commit-id: 7ef53646c119323e383a694a7462a1e4123e0003
Former-commit-id: c0f23064587e74ae00b25c66530274c912c386a9
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox