Fix PartialSimpleIdealGasMedium_setState_psX

[RaphaelGebhart]

Closes #4749 .

Test model:

model PartialSimleIdealGasMediumTest "Checks Modelica.Media.Air.SimpleAir.setState_psX"

  replaceable package Medium = Modelica.Media.Air.SimpleAir;
  Medium.AbsolutePressure p=pressureSawTooth.y;
  Medium.Temperature T=temperatureSawTooth.y;
  Medium.SpecificEntropy s = Modelica.Media.Air.SimpleAir.specificEntropy_pTX(p,T, Medium.X_default);
  Medium.ThermodynamicState state = Medium.setState_psX(p,s); // This requires modification
  Medium.Temperature T1 = Medium.temperature_ps(p,s);

  Boolean check = abs(T-T1) < Modelica.Constants.eps; // Check passed

  Modelica.Blocks.Sources.SawTooth pressureSawTooth(
    amplitude=2e5,
    period=0.5,
    offset=0.5e5,
    startTime=0.1) annotation (Placement(transformation(extent={{-10,12},{10,32}})));
  Modelica.Blocks.Sources.SawTooth temperatureSawTooth(
    amplitude=200,
    period=0.5,
    offset=273.15,
    startTime=0.2) annotation (Placement(transformation(extent={{-10,-30},{10,-10}})));

end PartialSimleIdealGasMediumTest;