BUG: extract_metric: fixed missing initialization
BUG: extract_metric: fixed missing initialization

Former-commit-id: ed7a347cf9dfb9dae1d84af5e89b2be463d9389e
Former-commit-id: dcb69be65769132dd0f2fbaa29e923a8780ade85
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox