fix lockless io-stealing
The uninitialized continuation pointer may be returned without being
written resulting in dispatching an invalid fiber pointer.
If a CQE is stolen but no fiber is blocked on the corresponding future
IoContext::getContinuationsFromCompletions will call future->completeAndGetContinuation
which will set the future's completion but will not return a continuation.
If no continuation is returned from Future::completeAndGetContinuation
the continuation buffer is not advanced by IoContext::getContinuationsFromCompletions (emper/io/IoContext.hpp:259).
IoContext::reapSingleCompletion does not check if a continuation was
created and assumed that continuation contains a valid Fiber* if
a CQE was successfully stolen.
Initialize continuation with nullptr, which is a valid Fiber*.