BUG: MGridGenAgglomerate: make sure face data is correct size

src/fvAgglomerationMethods/MGridGenGamgAgglomeration/MGridGenGAMGAgglomeration.C

@@ -59,7 +59,11 @@ Foam::MGridGenGAMGAgglomeration::MGridGenGAMGAgglomeration
 
     // Start geometric agglomeration from the cell volumes and areas of the mesh
     scalarField* VPtr = const_cast<scalarField*>(&fvMesh_.cellVolumes());
-    vectorField* SfPtr = const_cast<vectorField*>(&fvMesh_.faceAreas());
+    SubField<vector> Sf(fvMesh_.faceAreas(), fvMesh_.nInternalFaces());
+    vectorField* SfPtr = const_cast<vectorField*>
+    (
+        &Sf.operator const vectorField&()
+    );
 
     // Create the boundary area cell field
     scalarField* SbPtr(new scalarField(fvMesh_.nCells(), 0));