asIJITCompilerV2 implementations can't always track function destruction

Per the docs:

The difference is that for version 2 it is optional to provide the JIT function at [the time of NewFunction].

Except that functions may be destroyed before you ever assign them a JIT function.
CleanFunction is never called if the JIT function pointer is null. Perhaps it should be?

AFAICT, without assigning a dummy jitFunction the JIT v2 interface doesn't let you hold onto functions safely.