ActiveAE: set the scaled volume to zero if the volume is zero

@fritsch helped me to find out the solution to the issue that the zero volume isn't a real zero volume. As he pointed out we need to check if m_volume > 0.0 and only then do something here otherwise we have at zero volume:

PercentToGain(0.0) = -60;

GainToScale(-60) = pow(10.0f, -3) = 1.0 / (10^3) = 1.0 / 1000 = 0.001

and  0.001 != 0.0f