Core/Vehicles: Fixed a crash that happened if someone called ExitVehicle during seat changing

src/server/game/Entities/Unit/Unit.cpp

@@ -16605,7 +16605,12 @@ void Unit::ChangeSeat(int8 seatId, bool next)
     // Todo: the functions below could be consolidated and refactored to take
     // SeatMap::const_iterator as parameter, to save redundant map lookups.
     m_vehicle->RemovePassenger(this);
-    if (!m_vehicle->AddPassenger(this, seatId))
+
+    // Set m_vehicle to NULL before adding passenger as adding new passengers is handled asynchronously
+    // and someone may call ExitVehicle again before passenger is added to new seat
+    Vehicle* veh = m_vehicle;
+    m_vehicle = NULL;
+    if (!veh->AddPassenger(this, seatId))
         ASSERT(false);
 }