Prevent crash when accessing Node Multiplayer from thread

[AThousandShips]

Prevents crashes when accessing a null-pointer on thread check failing in SceneTree, still won't prevent the same on access to multiplayer from script, where you still need to check for null

Didn't use ERR_FAIL_NULL because we already get an error from SceneTree so felt excessive to add another error message

Could alternatively add a check here for threading, but thought best to leave it in SceneTree to prevent confusion/divergence

Haven't tested since I don't have mono set up but might prevent a crash in #77707

[Faless]

I'm not super up to date regarding threading macros, but I think we could use:

ERR_READ_THREAD_GUARD_V(Ref<MultiplayerAPI>())

and

ERR_READ_THREAD_GUARD_V(ERR_LOCKED)

cc @RandomShaper

[AThousandShips]

My reasoning here was to leave the error messages to the SceneTree side to avoid duplication and match changes over there, but can easily add checks directly here, though it'll mean checks run twice

[Faless]

Ah, that makes sense!

[Faless]

LGTM 👍 🏆

[RandomShaper]

I think the reasoning about the checks makes sense.

[YuriSizov]

Thanks!

[AThousandShips]

Thank you!

[YuriSizov]

Cherry-picked for 4.1.2.