Fixed a crash when city removal left a unit stranded RM#81

https://redmine.freeciv.org/issues/81

author: @cazfi
(nullptr changed to NULL by me)

freeciv/freeciv/server/citytools.c

@@ -3464,14 +3464,17 @@ void city_landlocked_sell_coastal_improvements(struct tile *ptile)
 ****************************************************************************/
 void city_refresh_vision(struct city *pcity)
 {
-  v_radius_t vision_radius_sq = V_RADIUS(
-   (short int) get_city_bonus(pcity, EFT_CITY_VISION_RADIUS_SQ, V_MAIN),
-   (short int) get_city_bonus(pcity, EFT_CITY_VISION_RADIUS_SQ, V_INVIS),
-   (short int) get_city_bonus(pcity, EFT_CITY_VISION_RADIUS_SQ, V_SUBSURFACE)
-  );
-
-  vision_change_sight(pcity->server.vision, vision_radius_sq);
-  ASSERT_VISION(pcity->server.vision);
+  if (pcity->server.vision != NULL) {
+
+    v_radius_t vision_radius_sq = V_RADIUS(
+    (short int) get_city_bonus(pcity, EFT_CITY_VISION_RADIUS_SQ, V_MAIN),
+    (short int) get_city_bonus(pcity, EFT_CITY_VISION_RADIUS_SQ, V_INVIS),
+    (short int) get_city_bonus(pcity, EFT_CITY_VISION_RADIUS_SQ, V_SUBSURFACE)
+    );
+
+    vision_change_sight(pcity->server.vision, vision_radius_sq);
+    ASSERT_VISION(pcity->server.vision);
+  }
 }
 
 /************************************************************************//**
@@ -3576,8 +3579,11 @@ bool city_map_update_radius_sq(struct city *pcity)
     city_refresh_vision(pcity);
   }
 
-  /* if city is under AI control update it */
-  adv_city_update(pcity);
+  /* City removal might be ongoing, and advisor data already deleted */
+  if (pcity->server.adv != NULL) {
+    /* If city is under AI control, update it */
+    adv_city_update(pcity);
+  }
 
   notify_player(city_owner(pcity), city_tile(pcity), E_CITY_RADIUS_SQ,
                 ftc_server, _("The size of the city map of %s is %s."),