HAL_Linux: fixed functor comparison

libraries/AP_HAL_Linux/Scheduler.cpp

@@ -250,7 +250,7 @@ void LinuxScheduler::_run_timers(bool called_from_timer_thread)
     }
     // now call the timer based drivers
     for (int i = 0; i < _num_timer_procs; i++) {
-        if (_timer_proc[i] != NULL) {
+        if (_timer_proc[i]) {
             _timer_proc[i]();
         }
     }
@@ -299,7 +299,7 @@ void LinuxScheduler::_run_io(void)
 
     // now call the IO based drivers
     for (int i = 0; i < _num_io_procs; i++) {
-        if (_io_proc[i] != NULL) {
+        if (_io_proc[i]) {
             _io_proc[i]();
         }
     }