Previously, the following piece of code would compile but then enter a
loop, typically leading to a segmentation fault

  Dune::FieldMatrix<double, 2, 3> A;
  // populate A

  Dune::DynamicMatrix<double> B_good(3, 3);
  // populate B_good
  A.rightmultiply(B_good);

This would also happen for the following illegal extension of the above code

  Dune::FieldMatrix<double, 3, 2> B_bad;
  // populate B_bad
  A.rightmultiply(B_bad);

The former now works as expected while the latter fails an assertion.