FIX: replaced temp internalField with DimensionedField::null (fixes #3082)

GeometricField::Boundary allocated using createZeroBoudnaryPtr held a dangling reference to a temporary object (see also #3082 (closed)).

This dangling reference was never accessed, so it didn't cause any wrong memory access. It did however increase the peak memory consumption.

Replaced the temporary object with DimensionedField<Type, volMesh>::null

src/optimisation/adjointOptimisation/adjoint/global/boundaryFieldsFwd.H

@@ -50,13 +50,13 @@ namespace Foam
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
-//volFields
+//- volFields
 typedef typename volScalarField::Boundary boundaryScalarField;
 typedef typename volVectorField::Boundary boundaryVectorField;
 typedef typename volTensorField::Boundary boundaryTensorField;
 
-//pointFields - actually a plain list of fields with dimension equal to the
-// number of points per patch
+//- pointFields - actually a plain list of fields with dimension equal to the
+//- number of points per patch
 typedef List<Field<scalar>> pointBoundaryScalarField;
 typedef List<Field<vector>> pointBoundaryVectorField;
 typedef List<Field<tensor>> pointBoundaryTensorField;