Fix: check for the existence of shadow and rotor vehicles for aircraft

Instead of just assuming that it exists in the savegame that got loaded.

src/saveload/vehicle_sl.cpp

@@ -454,13 +454,17 @@ void AfterLoadVehicles(bool part_of_load)
 				if (Aircraft::From(v)->IsNormalAircraft()) {
 					v->GetImage(v->direction, EIT_ON_MAP, &v->sprite_cache.sprite_seq);
 
-					/* The plane's shadow will have the same image as the plane, but no colour */
+					/* The aircraft's shadow will have the same image as the aircraft, but no colour */
 					Vehicle *shadow = v->Next();
+					if (shadow == nullptr) SlErrorCorrupt("Missing shadow for aircraft");
+
 					shadow->sprite_cache.sprite_seq.CopyWithoutPalette(v->sprite_cache.sprite_seq);
 
 					/* In the case of a helicopter we will update the rotor sprites */
 					if (v->subtype == AIR_HELICOPTER) {
 						Vehicle *rotor = shadow->Next();
+						if (rotor == nullptr) SlErrorCorrupt("Missing rotor for helicopter");
+
 						GetRotorImage(Aircraft::From(v), EIT_ON_MAP, &rotor->sprite_cache.sprite_seq);
 					}