macOS KVM: guard the IPC socket sinks against a stopped user LaunchAgent

[PTR-inc]

Summary

One of the test setups is running the root and user agent through terminal sessions, which showed the uncaught: 'cannot read property write of undefined' error after the user agent died and I tried to connect to the desktop.

What's happening is kvm_relay_setup() returns NULL when /tmp/meshagent-kvm-.sock is still on disk but the user agent is stopped, so nothing accepts on it. getRemoteDesktop() kept that NULL in ptrs->kvmPipe, and KVM_IPC_SOCKET is only set for the LoginWindow case, so the first desktop byte read "write" off undefined in the write sink. No protected call sits above a sink, so Duktape made it fatal and the root service exited 254 (ILIBCRITICALEXITMSG) with: uncaught: 'cannot read property write of undefined'

The pause and resume sinks had the same unguarded dereference.

Fixed by checking that KVM_IPC_SOCKET is an object in all three, return ILibTransport_DoneState_ERROR from the write sink, and report the failed connect to the MeshCentral console.

Details

• 🧠 I used LLMs/AI in this contribution and reviewed all generated content.
  I understand that I am responsible for and able to explain every line of code I submit.
• 🛠️ I have self-reviewed my code and self-tested it against a MeshCentral server to ensure it works as expected.
• 🖥️ My change compiles on every platform it affects (Windows / Linux / macOS / FreeBSD), and I have considered
  the impact on platforms and architectures I could not test.
• 📦 If I changed JavaScript modules under modules/, I re-embedded them so the compiled-in copies in
  microscript/ILibDuktape_Polyfills.c match (the agent runs the embedded copies, not the files on disk).
• 🤖 I ran the agent self-test where appropriate (see "Self Test" in readme.md).
• 📄 Documentation updates are included (if applicable), e.g. the .msh options table in readme.md.
• 🧰 Updates to vendored dependencies (OpenSSL, zlib, ...) are listed and explained.
• ⚠️ CI passes and is green (Windows / Linux / macOS / FreeBSD builds and CodeQL).

Testing

Tested on macos arm and x64