GEAR stars : fix negative initial metallicity
This ensures the stars do not have negative metallicities when GEARChemistry:initial_metallicity < 0 and hence non-physical lifetimes. This is already implemented for the gas but not the stars.
This ensures the stars do not have negative metallicities when GEARChemistry:initial_metallicity < 0 and hence non-physical lifetimes. This is already implemented for the gas but not the stars.