t/helper/ref-store: initialize oid in resolve-ref

t/helper/ref-store: initialize oid in resolve-ref

This will print $ZERO_OID when asking for a non-existent ref from the
test-helper.

Since resolve-ref provides direct access to refs_resolve_ref_unsafe(), it
provides a reliable mechanism for accessing REFNAME, while avoiding the implicit
resolution to refs/heads/REFNAME.

Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>