SITL: ensure we don't run the sitl timer twice

this caused problems with random()

libraries/Desktop/support/sitl.cpp

@@ -223,7 +223,12 @@ static void sitl_simulator_output(void)
 static void timer_handler(int signum)
 {
 	static uint32_t last_update_count;
+	static bool running;
 
+	if (running) {
+		return;
+	}
+	running = true;
 	cli();
 
 #ifndef __CYGWIN__
@@ -261,11 +266,13 @@ static void timer_handler(int signum)
 	if (update_count == 0) {
 		sitl_update_gps(0, 0, 0, 0, 0, false);
 		sei();
+		running = false;
 		return;
 	}
 
 	if (update_count == last_update_count) {
 		sei();
+		running = false;
 		return;
 	}
 	last_update_count = update_count;
@@ -280,6 +287,7 @@ static void timer_handler(int signum)
 	sitl_update_barometer(sim_state.altitude);
 	sitl_update_compass(sim_state.rollDeg, sim_state.pitchDeg, sim_state.heading);
 	sei();
+	running = false;
 }