[amg][bugfix] Fix dirichlet processing in FastAMG

The comment and surroundig code indicate that FastAMG should
solve during pre() for dirichlet entries. This is determined
by checking if a row does contain a diagonal entry while
all other entries are zero.

The check for this was buggy because hasDiagonal was always false.
Now it's set to true if a diagonal entry is found and nonzero.

dune/istl/paamg/fastamg.hh

@@ -471,7 +471,7 @@ namespace Dune
         for(ColIter col=row->begin(); col!=row->end(); ++col) {
           if(row.index()==col.index()) {
             diag = col;
-            hasDiagonal = false;
+            hasDiagonal = (*col != zero);
           }else{
             if(*col!=zero)
               isDirichlet = false;