Fixes#10020
Previous fix 445e1d3 tried to close one double invocation, but added
another, since it failed to ensure all potential nullings of the opt
shared_future happened before a new allocator could reset it.
This simplifies the code by making clearing the shared_future a
pre-requisite for resolving its contents (as read by waiters).
Also removes any need for try-catch etc.
Closes#10024
1e66043412