multicomponentThermo: Fix for singleComponentMixture

This prevents a segfault when using singleComponentMixture and no mass
fraction fields are stored

Resolves bug report https://bugs.openfoam.org/view.php?id=4006

src/thermophysicalModels/multicomponentThermo/include/GeometricFieldListSlicer.H

@@ -74,21 +74,29 @@ public:
         inline GeometricFieldListSlicer()
         {}
 
-        //- Construct from a list of fields
+        //- Construct from a mesh and a list of fields
-        inline GeometricFieldListSlicer(const PtrList<geoFieldType>& geoFields)
+        inline GeometricFieldListSlicer
+        (
+            const typename geoFieldType::Mesh& mesh,
+            const PtrList<geoFieldType>& geoFields
+        )
         {
-            set(geoFields);
+            set(mesh, geoFields);
         }
 
 
     // Member Functions
 
         //- Set the field pointers
-        inline void set(const PtrList<geoFieldType>& geoFields)
+        inline void set
+        (
+            const typename geoFieldType::Mesh& mesh,
+            const PtrList<geoFieldType>& geoFields
+        )
         {
             fields_.resize(geoFields.size());
 
-            patchFields_.resize(geoFields[0].boundaryField().size(), fields_);
+            patchFields_.resize(mesh.boundary().size(), fields_);
 
             forAll(geoFields, fieldi)
             {

src/thermophysicalModels/multicomponentThermo/multicomponentThermo/multicomponentThermoI.H

@@ -107,7 +107,7 @@ inline Foam::label Foam::multicomponentThermo::specieIndex
 inline Foam::volScalarFieldListSlicer
 Foam::multicomponentThermo::implementation::Yslicer() const
 {
-    return volScalarFieldListSlicer(Y_);
+    return volScalarFieldListSlicer(mesh(), Y_);
 }
 
 

src/thermophysicalModels/multicomponentThermo/psiuMulticomponentThermo/psiuMulticomponentThermoI.H

@@ -58,7 +58,7 @@ inline const Foam::volScalarField& Foam::psiuMulticomponentThermo::Y
 inline Foam::volScalarFieldListSlicer
 Foam::psiuMulticomponentThermo::implementation::Yslicer() const
 {
-    return volScalarFieldListSlicer(Y_);
+    return volScalarFieldListSlicer(mesh(), Y_);
 }