Core/Vehicles: Fix a crash in VehicleJoinEvent::Execute

Crash would happen if multiple events were scheduled for the same seat.
Fixes #9255
Core/Vehicles: Make RemovePendingEventsForSeat iterating crashsafe
Core/Vehicles: Really make RemovePendingEventsForSeat iterating crash safe
Merge branch 'master' of https://github.com/TrinityCore/TrinityCore