[PL/THM] Explicitly set phi_fr_prev, don't update

The phi_fr_prev variable initialization was not correct.
Setting it explicitly avoids any non-initialized states.

ProcessLib/ThermoHydroMechanics/IntegrationPointData.h

@@ -71,15 +71,14 @@ struct IntegrationPointData final
         DisplacementDim>::MaterialStateVariables>
         material_state_variables;
 
-    double phi_fr = 0;
-    double phi_fr_prev;
+    double phi_fr = std::numeric_limits<double>::quiet_NaN();
+    double phi_fr_prev = std::numeric_limits<double>::quiet_NaN();
     double integration_weight;
 
     double porosity;
 
     void pushBackState()
     {
-        phi_fr_prev = phi_fr;
         eps0_prev = eps0;
         eps_prev = eps;
         eps_m_prev = eps_m;

ProcessLib/ThermoHydroMechanics/ThermoHydroMechanicsFEM-impl.h

@@ -389,7 +389,15 @@ ConstitutiveRelationsValues<DisplacementDim> ThermoHydroMechanicsLocalAssembler<
                     vars, MaterialPropertyLib::Variable::temperature,
                     x_position, t, dt);
 
-        double const& phi_fr_prev = ip_data.phi_fr_prev;
+        double const phi_fr_prev = [&]()
+        {
+            MaterialPropertyLib::VariableArray vars_prev;
+            vars_prev.temperature = T_prev_int_pt;
+            return (*medium)[MaterialPropertyLib::PropertyType::volume_fraction]
+                .template value<double>(vars_prev, x_position, t, dt);
+        }();
+        ip_data.phi_fr_prev = phi_fr_prev;
+
         // alpha_T^I
         MathLib::KelvinVector::KelvinVectorType<
             DisplacementDim> const ice_linear_thermal_expansion_coefficient =