s3: smbd: Don't overwrite contents of fsp->aio_requests[0] with NULL via TALLOC_FREE().

s3: smbd: Don't overwrite contents of fsp->aio_requests[0] with NULL via TALLOC_FREE().

They may have been carefully set by the aio_del_req_from_fsp()
destructor so we must not overwrite here.

Found via some *amazing* debugging work from Ashok Ramakrishnan <aramakrishnan@nasuni.com>.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14515

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Wed Sep 30 11:18:43 UTC 2020 on sn-devel-184