Fixed MoistAir.T_psX for size(X,1) not= nX

[RaphaelGebhart]

Closes #4771.
Test example:

model MoistAirEntropy "Tests MoistAir.setState_psX()"

  replaceable package Medium = Modelica.Media.Air.MoistAir;
  Medium.AbsolutePressure p=pressureSawTooth.y;//Medium.p_default;
  Medium.Temperature T=temperatureSawTooth.y;//Medium.T_default;
  Medium.MassFraction Xi[:]={massFractionSawTooth.y};// Medium.X_default[1:Medium.nXi];
  Medium.MassFraction X[:] = {Xi[1], 1-Xi[1]}; // Medium.X_default;

  Medium.SpecificEntropy s1 = Medium.specificEntropy_pTX(p,T,Xi);
  Medium.SpecificEntropy s2 = Medium.specificEntropy_pTX(p,T,X);
  Boolean check1 = abs(s1-s2) < Modelica.Constants.eps; // Check passed

  Medium.ThermodynamicState state1 = Medium.setState_psX(p,s1,Xi); // This requires modification
  Medium.ThermodynamicState state2 = Medium.setState_psX(p,s1,X);
  Medium.Temperature T1 = Medium.temperature(state1);
  Medium.Temperature T2 = Medium.temperature(state2);
  Boolean check2 = abs(T1-T) < 1e-12; // Check passed
  Boolean check3 = abs(T2-T) < 1e-12; // Check passed

  Modelica.Blocks.Sources.SawTooth pressureSawTooth(
    amplitude=2e5,
    period=0.5,
    offset=0.5e5,
    startTime=0.1) annotation (Placement(transformation(extent={{-80,42},{-60,62}})));
  Modelica.Blocks.Sources.SawTooth temperatureSawTooth(
    amplitude=200,
    period=0.5,
    offset=273.15,
    startTime=0.2) annotation (Placement(transformation(extent={{-80,0},{-60,20}})));
  Modelica.Blocks.Sources.SawTooth massFractionSawTooth(
    amplitude=0.5,
    period=0.5,
    offset=0,
    startTime=0.3) annotation (Placement(transformation(extent={{-80,-40},{-60,-20}})));
end MoistAirEntropy;

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