Merge branch 'fix-amg-empty-matrix-on-one-proc' into 'master'

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

See merge request !159

dune/istl/bcrsmatrix.hh

@@ -927,6 +927,9 @@ namespace Dune {
           DUNE_THROW(BCRSMatrixError,"creation only allowed for uninitialized matrix");
         if(Mat.build_mode!=row_wise)
           DUNE_THROW(BCRSMatrixError,"creation only allowed if row wise allocation was requested in the constructor");
+        if(i==0 && _Mat.N()==0)
+          // empty Matrix is always built.
+           Mat.ready = built;
       }
 
       //! prefix increment

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();