fix CheckIfDiagonalPresent for scalar valued matrices

Nils-Arne Dreierrequested to merge
nils.dreier/dune-istl:fix_checkIfDiagonalPresent_for_scalar_values_matrices into master

This MR fixes CheckIfDiagonalPresent for scalar matrices, which is called by several preconditioners if dune-istl is compiled with DUNE_ISTL_WITH_CHECKING.

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