Attempts to fix the thread_safe_callback flakiness. It's unclear what
the flake is about, the exit code is apparently `C0000005` or
`ACCESS_VIOLATION`, pointing to an issue with memory access. My only
guess is that maybe dropping the `Option<extern "C" fn ()>` is somehow
checking the validity of the function pointer and since the function has
been dropped, the pointer is no longer valid and sometimes points to
memory that should not be accessed.
So now the will explicitly drop the functions before they get
deallocated. If this doesn't fix the flake then something beyond my
understanding is wrong.
