[lua] Adjust mob add effect to account for edge cases

[WinterSolstice8]

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• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

No more healing for 4 trillion hp, see comments in code

Steps to test these changes

Get Qoofim to auto attack you until it damages you for < 50, not heals you for 4 trillion

[WinterSolstice8]

Apparently this is in line with item add effects. Item add effects can only generate a bonus, and no penalty. So at least we're consistent after this PR