C++11: Replaced the C NULL with the safer C++11 nullptr

Requires gcc version 4.7 or higher

applications/utilities/parallelProcessing/reconstructParMesh/reconstructParMesh.C

@@ -227,7 +227,7 @@ autoPtr<mapPolyMesh> mergeSharedPoints
 
     if (returnReduce(pointToMaster.size(), sumOp<label>()) == 0)
     {
-        return autoPtr<mapPolyMesh>(NULL);
+        return autoPtr<mapPolyMesh>(nullptr);
     }
 
     polyTopoChange meshMod(mesh);