Fixed premature background subrequest finalization. When "aio" or "aio threads" is used while processing the response body of an in-memory background subrequest, the subrequest could be finalized with an aio operation still in progress. Upon aio completion either parent request is woken or the old r->write_event_handler is called again. The latter may result in request errors. In either case post_subrequest handler is never called with the full response body, which is typically expected when using in-memory subrequests. Currently in nginx background subrequests are created by the upstream module and the mirror module. The issue does not manifest itself with these subrequests because they are header-only. But it can manifest itself with third-party modules which create in-memory background subrequests.