SITL: correct adsb initialisation

Parameters have not yet been initialised in _sitl_setup, so move
should-run checks into update

libraries/AP_HAL_SITL/SITL_State.cpp

@@ -63,6 +63,8 @@ void SITL_State::_set_param_default(const char *parm)
  */
 void SITL_State::_sitl_setup(const char *home_str)
 {
+    _home_str = home_str;
+
 #ifndef __CYGWIN__
     _parent_pid = getppid();
 #endif
@@ -97,10 +99,6 @@ void SITL_State::_sitl_setup(const char *home_str)
             gimbal = new SITL::Gimbal(_sitl->state);
         }
 
-        if (_sitl->adsb_plane_count >= 0) {
-            adsb = new SITL::ADSB(_sitl->state, home_str);
-        }
-
         fg_socket.connect("127.0.0.1", 5503);
     }
 
@@ -173,6 +171,15 @@ void SITL_State::_fdm_input_step(void)
         _update_barometer(_sitl->state.altitude);
         _update_compass(_sitl->state.rollDeg, _sitl->state.pitchDeg, _sitl->state.yawDeg);
         _update_flow();
+
+        if (_sitl->adsb_plane_count >= 0 &&
+            adsb == nullptr) {
+            adsb = new SITL::ADSB(_sitl->state, _home_str);
+        } else if (_sitl->adsb_plane_count == -1 &&
+                   adsb != nullptr) {
+            delete adsb;
+            adsb = nullptr;
+        }
     }
 
     // trigger all APM timers.

libraries/AP_HAL_SITL/SITL_State.h

@@ -222,6 +222,8 @@ private:
     const char *_client_address;
 
     const char *defaults_path = HAL_PARAM_DEFAULTS_PATH;
+
+    const char *_home_str;
 };
 
 #endif // CONFIG_HAL_BOARD == HAL_BOARD_SITL