NetMQ Poller cleanup is not thread safe

[SenseYang]

Hi, I have observed frequent crashing of my app after adopting NetMQ especially when the Poller is being cleaned up. Error logs:

Unhandled exception. System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')
2025-07-25T22:26:11.8724090Z at System.Collections.Generic.List`1.get_Item(Int32 index)
2025-07-25T22:26:11.8724230Z at NetMQ.Core.SocketBase.ProcessTerm(Int32 linger)
2025-07-25T22:26:11.8724360Z at NetMQ.Core.Own.Terminate()
2025-07-25T22:26:11.8724485Z at NetMQ.Core.SocketBase.StartReaping(Poller poller)
2025-07-25T22:26:11.8724618Z at NetMQ.Core.Reaper.ProcessReap(SocketBase socket)
2025-07-25T22:26:11.8724748Z at NetMQ.Core.ZObject.ProcessCommand(Command cmd)
2025-07-25T22:26:11.8724868Z at NetMQ.Core.Reaper.InEvent()
2025-07-25T22:26:11.8724974Z at NetMQ.Core.Utils.Poller.Loop()
2025-07-25T22:26:11.8725162Z at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
2025-07-25T22:26:11.8725338Z --- End of stack trace from previous location ---
2025-07-25T22:26:11.8725505Z at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)

It seems SocketBase should at least add more thread safety checks, or handle termination exceptions better.