Resolve "Skynet Skyfile Upload Panic"

MERGE REQUEST

Overview

This MR fixes a bug where skyfile metadata exceeding the size of a sector would cause a uint64 underflow which caused a panic when creating a slice.

Example for Visual Changes

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

Closes #3948 (closed)