prevent target_system=0 in toggle safety switch (#3269)

a user tried to toggle safety on an antenna tracker and MissionPlanner
sent it to target_system=0, which resulted in toggling safety on a
flying aircraft, causing it to crash

GCSViews/FlightData.cs

@@ -1692,8 +1692,13 @@ private void BUTactiondo_Click(object sender, EventArgs e)
                     }
                     if (CMB_action.Text == actions.Toggle_Safety_Switch.ToString())
                     {
+                        var target_system = (byte)MainV2.comPort.sysidcurrent;
+                        if (target_system == 0) {
+                            log.Info("Not toggling safety on sysid 0");
+                            return;
+                        }
                         var custom_mode = (MainV2.comPort.MAV.cs.sensors_enabled.motor_control && MainV2.comPort.MAV.cs.sensors_enabled.seen) ? 1u : 0u;
-                        var mode = new MAVLink.mavlink_set_mode_t() { custom_mode = custom_mode, target_system = (byte)MainV2.comPort.sysidcurrent };
+                        var mode = new MAVLink.mavlink_set_mode_t() { custom_mode = custom_mode, target_system = target_system };
                         MainV2.comPort.setMode(mode, MAVLink.MAV_MODE_FLAG.SAFETY_ARMED);
                         ((Control)sender).Enabled = true;
                         return;