Fix physics platform crash

[lawnjelly]

Physics body previously stored the RID of a collision object and accessed it on the next frame, leading to a crash if the object had been deleted. This PR checks the object still exists via the ObjectID prior to access.

Fixes 4.x version of #74732
4.x port of #88946

Notes

• See [3.x] Fix physics on_floor_body crash #88946 for details, and pros cons.
• Also note possibility of race condition if another thread deletes object in between checking ObjectDB and access.

[fabriceci]

I can't speak for the best approach, but the solution is simple and fixes the bug, despite its drawbacks. I'll let someone else choose the best approach.

[akien-mga]

Let's go with this for now and revisit a potential better / more core solution in the future.

[akien-mga]

Thanks!