AMG::pre: set `hasDiagonal` to `true` if diagonal entry exists

Thanks to Nils-Arne Dreier for noticing.

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;