rados.cc: fix pool alignment check

Only check pool alignment if io_ctx is initialized.

Introduced in 304b08a23a3db57010078046955a786fe3589ef8
Fixes: #8652
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2020 from ceph/wip-fix-rados-tool

rados.cc: fix pool alignment check

Reviewed-by: Sage Weil <sage@inktank.com>