Make MassMoment.GetCenterOfMass return 0 if mass is 0
Make MassMoment.GetCenterOfMass return 0 if mass is 0

Previous I was returning NaN. Technically the center of mass is
undefined when mass is 0, but returning NaN tends to cause cascading
NaNs whereas any non-NaN number would later get cancelled by the zero
mass.
virtuallynaked/virtually-naked,virtuallynaked/virtually-naked