use reset() instead of assignement from NULL to reset a shared_ptr, as...

use reset() instead of assignement from NULL to reset a shared_ptr, as apparently only very new compilers allow the latter

[[Imported from SVN: r6430]]

dune/solvers/iterationsteps/multigridstep.hh

@@ -112,7 +112,7 @@
 
             for (int i=0; i<int(mat_.size())-1; i++)
             {
-                mat_[i] = NULL;
+                mat_[i].reset();
                 ignoreNodesHierarchy_[i] = NULL;
             }