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

Requires gcc version 4.7 or higher

src/OpenFOAM/db/dynamicLibrary/dlLibraryTable/dlLibraryTable.C

@@ -142,7 +142,7 @@ bool Foam::dlLibraryTable::close
 
         bool ok = dlClose(libPtrs_[index]);
 
-        libPtrs_[index] = NULL;
+        libPtrs_[index] = nullptr;
         libNames_[index] = fileName::null;
 
         if (!ok)
@@ -179,7 +179,7 @@ void* Foam::dlLibraryTable::findLibrary(const fileName& functionLibName)
     {
         return libPtrs_[index];
     }
-    return NULL;
+    return nullptr;
 }