rgw: fix crash in swift CORS preflight request

Fixes: #8586

This fixes error handling, in accordance with commit 6af5a537 that fixed
the same issue for the S3 case.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #2162 from ceph/wip-8586

rgw: fix crash in swift CORS preflight request

Reviewed-by: Sage Weil <sage@redhat.com>