SITL: fixed the pwm output on startup

libraries/Desktop/support/sitl.cpp

@@ -197,9 +197,12 @@ static void sitl_simulator_output(void)
 	last_update = millis();
 
 	for (i=0; i<11; i++) {
-		// the registers are 2x the PWM value
+		if (*reg[i] == 0xFFFF) {
+			pwm[i] = 0;
+		} else {
 			pwm[i] = (*reg[i])/2;
 		}
+	}
 
 	if (!desktop_state.quadcopter) {
 		// 400kV motor, 16V