Fix unlock panic in downloadDestinationWriter

MERGE REQUEST

Overview

Discord user fabwa submitted logs showing a panic in the download destination writer. fatal error: sync: unlock of unlocked mutex this MR should address that edge case.

Logs: sia_logs

Example for Visual Changes

Regression Test showing panic
https://asciinema.org/a/wKlfIaCnfybWdgSjHmFn3zIj4

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods or updated methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to workflow
• Issue added to Sia-UI repo for new supporting features
• Changelog File Created

Issues Closed