[ODE] dRandInt Crash Fix

[Colin Bonstead]

I recently fixed a crash in our game that was occurring in the ODE step. 
The crash typically only happened after the game had been sitting around 
running for 8 hours or so, but a few times it happened in less time than 
that.  It would blow up soon after the joints array was randomized 
during the step.  It turns out that somehow dRandInt was returning a 
number one greater than n, which caused the joint randomization to put 
an invalid pointer in the joint array.

I'm not sure how that could happen, but at this point I don't really 
have time to investigate it any further, just getting it fixed is more 
important.  Frankly, I'm not quite sure why dRandInt even needs to use 
doubles.  Wouldn't just doing dRand() % n give you a number between 0 
and n?  Regardless, here's a fixed version of dRandInt:

int dRandInt (int n)
{
  double a = double(n) / 4294967296.0;
  int ret = (int) (double(dRand()) * a);
  if (ret >= n)
	  ret = n-1;
  return ret;
}