Core/Vehicles: Fixed assertion failures related to seat changes, huge thanks to Machiavelli for well documented vehicle code and j4r0d for providing info how to reproduce the crash

src/server/game/Server/Protocol/Handlers/VehicleHandler.cpp

@@ -100,7 +100,7 @@ void WorldSession::HandleChangeSeatsOnControlledVehicle(WorldPacket &recv_data)
             {
                 if (Vehicle *vehicle = vehUnit->GetVehicleKit())
                     if (vehicle->HasEmptySeat(seatId))
-                        GetPlayer()->_EnterVehicle(vehicle, seatId);
+                        vehUnit->HandleSpellClick(GetPlayer(), seatId);
             }
             break;
         }
@@ -117,7 +117,7 @@ void WorldSession::HandleChangeSeatsOnControlledVehicle(WorldPacket &recv_data)
             else if (Unit *vehUnit = Unit::GetUnit(*GetPlayer(), guid))
                 if (Vehicle *vehicle = vehUnit->GetVehicleKit())
                     if (vehicle->HasEmptySeat(seatId))
-                        GetPlayer()->_EnterVehicle(vehicle, seatId);
+                        vehUnit->HandleSpellClick(GetPlayer(), seatId);
             break;
         }
         default: