Fix underflow for slow mobs and negative mobspeedmod

[MowFord]

I affirm:

• 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?

Troubleshooting Royal Jelly, i ran across a pretty silly bug. My local has MOB_SPEED_MOD = -10 and the princess jellies have speed of 2... so they were underflow zooming to the center

Steps to test these changes

Set map setting for mob_speed_mod negative, give a mob a speed such that this would be negative.

before:
https://imgur.com/f5eao3Q

after:
https://imgur.com/OmgTfXv

[TeoTwawki]

good 'nuff methinks