Fix Ythotha being able to move backwards

[lL1l1]

Issue

Ythotha can walk backwards (with 2.5 speed at that) which is obviously overpowered and also looks very weird.

chicken.backwards.walk.mp4

Description of the proposed changes

Set its reverse speed to 0 so it cannot walk backwards.

Testing done on the proposed changes

Spawn it and confirm it cannot walk backwards when given move orders near close behind it.

   CreateUnitAtMouse('xsl0401', 0,    0.00,    0.00,  0)

Additional context

BackUpDistance gets a default value from the engine, but I'm not sure how it is calculated, so I didn't add a comment in the annotation about it.

Checklist

• Changes are annotated, including comments where useful
• Changes are documented in the changelog for the next game version

[Basilisk3]

Tested and works. This is an important change, as the bug makes the Ythotha too powerful. Setting the BackUpDistance to 0 also fixed the bug for me, but I am not sure if it is the preferred solution.