Fix make-it-rain bug

[hansieodendaal]

Description

Fixed make-it-rain bug

Motivation and Context

The delay calculation was done using u64; it should be done with i64 to prevent wrap around.

How Has This Been Tested?

As per the original base node wallet implementation

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Feature refactor (No new feature or functional changes, but performance or technical debt improvements)
• New Tests
• Documentation

Checklist:

• I'm merging against the development branch.
• I ran cargo-fmt --all before pushing.
• I ran cargo test successfully before submitting my PR.
• I have squashed my commits into a single commit.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.