race in Future destructor

race in Future destructor

Summary:
@mnd wrote some Wangle code that would trip up with a `bad_function_call` exception. This Shouldn't Happen™ but the exception comes from trying to call a `std::function` which is null. We pretty thoroughly examined his usage and didn't find any problems, and this patch seems to make the error go away. See #4207781 for more details.

And reasoning about it, it makes sense. Inline comments explain the race.

Test Plan: Alas, I haven't been able to get a minimal repro and therefore a regression unit test. It's a hard race to trigger. I still don't understand why Matt's code does it.

Reviewed By: davejwatson@fb.com

FB internal diff: D1304001