[superlu] Ensure vectors x and b handed to 'apply' have same size

Similar to commit e8d16384 which
added the same for UMFPack.
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.

(cherry picked from commit 1a2f4d0f)
Signed-off-by: Steffen Müthing <muething@dune-project.org>

dune/istl/superlu.hh

@@ -561,6 +561,10 @@ namespace Dune
   void SuperLU<BCRSMatrix<FieldMatrix<T,n,m>,A> >
   ::apply(domain_type& x, range_type& b, InverseOperatorResult& res)
   {
+    if (mat.N() != b.size())
+      DUNE_THROW(ISTLError, "Size of right-hand-side vector b does not match the number of matrix rows!");
+    if (mat.M() != x.size())
+      DUNE_THROW(ISTLError, "Size of solution vector x does not match the number of matrix columns!");
     if(mat.M()+mat.N()==0)
       DUNE_THROW(ISTLError, "Matrix of SuperLU is null!");