aio: Avoid spurious coverity warning

aio: Avoid spurious coverity warning

PgAioResult.result is never accessed in the relevant path, but coverity
complains about an uninitialized access anyway. So just zero-initialize the
whole thing.  While at it, reduce the scope of the variable.

Reported-by: Ranier Vilela <ranier.vf@gmail.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/CAEudQApsKqd-s+fsUQ0OmxJAMHmBSXxrAz3dCs+uvqb3iRtjSw@mail.gmail.com