Replace NULL with nullptr or null in webrtc/base/.

webrtc/base/sigslot.h

 // sigslot.h: Signal/Slot classes
 //
 // Written by Sarah Thompson (sarah@telergy.com) 2002.
 //
 // License: Public domain. You are free to use this code however you like, with the proviso that
 //          the author takes on no responsibility or liability for any use.
 //
 // QUICK DOCUMENTATION
 //
 //                              (see also the full documentation at http://sigslot.sourceforge.net/)
@@ skipping 198 matching lines @@
                         pthread_mutex_unlock(get_mutex());
                 }
 
         private:
                 static pthread_mutex_t* get_mutex();
         };
 
         class multi_threaded_local
         {
         public:
-                multi_threaded_local()
-                {
-                        pthread_mutex_init(&m_mutex, NULL);
-                }
-                multi_threaded_local(const multi_threaded_local&)
-                {
-                        pthread_mutex_init(&m_mutex, NULL);
+                multi_threaded_local() { pthread_mutex_init(&m_mutex, nullptr); }
+                multi_threaded_local(const multi_threaded_local&) {
+                        pthread_mutex_init(&m_mutex, nullptr);
                 }
                 ~multi_threaded_local()
                 {
                         pthread_mutex_destroy(&m_mutex);
                 }
                 void lock()
                 {
                         pthread_mutex_lock(&m_mutex);
                 }
                 void unlock()
@@ skipping 419 matching lines @@
 
         template<typename A1, typename A2, typename A3, typename A4, typename A5, typename A6, typename A7, typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
         using signal7 = signal_with_thread_policy<mt_policy, A1, A2, A3, A4, A5, A6, A7>;
 
         template<typename A1, typename A2, typename A3, typename A4, typename A5, typename A6, typename A7, typename A8, typename mt_policy = SIGSLOT_DEFAULT_MT_POLICY>
         using signal8 = signal_with_thread_policy<mt_policy, A1, A2, A3, A4, A5, A6, A7, A8>;
 
 } // namespace sigslot
 
 #endif // WEBRTC_BASE_SIGSLOT_H__