gincostestimate and hypothetical indexes

Hello,

I figured out that it's not possible to use a hypothetical gin index, as
the gincostestimate function try to retrieve some statistical data from
the index meta page.

Attached patch fixes this. I believe this should be back-patched as was
a2095f7fb5a57ea1794f25d029756d9a140fd429.

Regards.
--
Julien Rouhaud
http://dalibo.com - http://dalibo.org