Forces code fix

[Gervase Lam]

Attached is a fix to a problem in the forces code.  With my experimental 
Bomber code, this could be seen when the Armageddon button was pressed 
when there are lots on Pingus around.  Some of the Pingus fell up the 
screen.

I will soon start a 'proper' version of the "Bomber" stuff as I have had 
no comments about the experimental Bomber code I posted.

Thanks,
Gervase.

pingus.200210122225.cvs.diff
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