[Bugfix] calling invert on zero matrix

Summary

Thus MR fixes in issue in the matrixtest. The function testMatrix(matrix,...) modifies the content of the passed matrix argument and sets it to 0. Inversion of the matrix after calling this function thus results in an error "Matrix is singular". See failing CI job https://gitlab.dune-project.org/core/dune-istl/-/jobs/630215

added bug label

requested review from @oliver.sander

assigned to @simon.praetorius

approved this merge request

enabled an automatic merge when all merge checks for fe0af10e pass

The failing pipeline is related to the bug in multirhstest and is investigated in !596 (merged)

canceled the automatic merge

merged

mentioned in commit 6972fdef