[!236] AMG::pre: set `hasDiagonal` to `true` if diagonal entry exists

Merge branch '55-amg-set-hasDiagonal-to-true' into 'master'

ref:core/dune-istl Thanks to Nils-Arne Dreier for noticing.

Closes: [#55]

See merge request [!236]

  [#55]: gitlab.dune-project.org/NoneNone/issues/55
  [!236]: gitlab.dune-project.org/core/dune-istl/merge_requests/236


Closes #55

dune/istl/paamg/amg.hh

@@ -577,7 +577,7 @@ namespace Dune
         for(ColIter col=row->begin(); col!=row->end(); ++col) {
           if(row.index()==col.index()) {
             diagonal = *col;
-            hasDiagonal = false;
+            hasDiagonal = true;
           }else{
             if(*col!=zero)
               isDirichlet = false;