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

Requires gcc version 4.7 or higher

src/OSspecific/POSIX/timer.C

@@ -118,7 +118,7 @@ Foam::timer::~timer()
         oldTimeOut_ = 0;
 
         // Restore signal handler
-        if (sigaction(SIGALRM, &oldAction_, NULL) < 0)
+        if (sigaction(SIGALRM, &oldAction_, nullptr) < 0)
         {
             FatalErrorInFunction
                 << "sigaction(SIGALRM) error"