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

Requires gcc version 4.7 or higher

src/OpenFOAM/db/functionObjects/functionObjectList/functionObjectList.C

@@ -606,7 +606,7 @@ bool Foam::functionObjectList::read()
                 {
                     // Delete the disabled functionObject
                     delete objPtr;
-                    objPtr = NULL;
+                    objPtr = nullptr;
                     continue;
                 }
             }

src/OpenFOAM/db/functionObjects/functionObjectList/functionObjectList.H

@@ -88,7 +88,7 @@ class functionObjectList
 
         //- Remove and return the function object pointer by name,
         //  and returns the old index via the parameter.
-        //  Returns a NULL pointer (and index -1) if it didn't exist
+        //  Returns a nullptr pointer (and index -1) if it didn't exist
         functionObject* remove(const word&, label& oldIndex);
 
         //- Search the specified directory for functionObject