Fix segmentation fault in AMG if matrix of one rank is empty.

For this we need to mark a matrix without rows built in row_wise mode as built once the CreateIterator is constructed and correct the number of vertices for a Subgraph of a MatrixGraph with 0 vertices.

This should fix the segementatio fault Kumar is experiencing.

dune/istl/paamg/graph.hh

@@ -1986,6 +1986,11 @@ namespace Dune
 
       VertexDescriptor* edge=edges_;
 
+      // Cater for the case that there are no vertices.
+      // Otherwise endVertex_ will get 1 below.
+      if ( graph.noVertices() == 0)
+        return;
+
       typedef typename Graph::ConstVertexIterator Iterator;
       Iterator endVertex=graph.end();