Make sure the vectors x and b handed to the 'apply' method have the correct size

This avoids strange bugs when using the setSubMatrix method: that method
automatically removes rows and columns from the matrix, and expects _the user_
to do the same for the x and b vectors.

dune/istl/umfpack.hh

@@ -314,6 +314,11 @@ namespace Dune {
      */
     virtual void apply(domain_type& x, range_type& b, InverseOperatorResult& res)
     {
+      if (umfpackMatrix_.N() != b.size())
+        DUNE_THROW(Dune::ISTLError, "Size of right-hand-side vector b does not match the number of matrix rows!");
+      if (umfpackMatrix_.M() != x.size())
+        DUNE_THROW(Dune::ISTLError, "Size of solution vector x does not match the number of matrix columns!");
+
       double UMF_Apply_Info[UMFPACK_INFO];
       Caller::solve(UMFPACK_A,
                     umfpackMatrix_.getColStart(),