Upstream: Ignore CommandType.Done with null Destination in SocketBase

[follesoe]

Patch

Commit `47bdc293` — Ignore processing CommandType.Done with null Destination (Thierry Matthey, 2024-07-29).

Files: `src/NetMQ/Core/SocketBase.cs` (3 lines)

Came in via merge commits `5714d52b` and `80831cec` from external branch `tmatthey/pr/check-destination-null` (author: Thierry Matthey at SLB / @tmatthey).

There was an earlier attempt (`7f9fb678` "If there is no destination ... just skip it.") that was reverted in `47b422d8`; the surviving `47bdc293` is the cleaner replacement.

What the patch does

Skips processing of `CommandType.Done` commands whose `Destination` field is null, instead of dereferencing it. Avoids a null-reference crash during socket shutdown when a Done command arrives after its target socket has already been collected.

Origin / sources

• External author: Thierry Matthey (SLB) — branch `tmatthey/pr/check-destination-null`
• Not yet submitted upstream as of last check.

Upstreaming work

• Get original repro from Thierry / SLB if possible — when does `Destination` come in null?
• Construct a deterministic test: shut down a socket while a peer has a pending Done command in flight, verify no NRE.
• Coordinate with @tmatthey on co-authorship for the upstream PR.
• Discuss with upstream whether the right fix is to skip silently, log, or fix the producer side.