Potential issue in listExtensions()

Hi,

Upstream commit d696406a9b255546bc1716d07199103abd8bb785 [1] added the
support for default extension version in \dx output and changed the query
that fetches the extension list.
The changed query seems problematic and might display duplicate entries of
extension if the same object oid exists in pg_description.

If I understand correctly, after oid wraparound, it is possible that the
existing oid from another catalog might be used again for other catalog
objects as per GetNewOidWithIndex().
If this is true, then it is possible that oid exists in pg_description for
an object will be used for extension oid, and we might get a duplicate
entry in \dx output.

```

*"FROM pg_catalog.pg_extension e " "LEFT JOIN pg_catalog.pg_namespace n ON
n.oid = e.extnamespace " "LEFT JOIN pg_catalog.pg_description d ON
d.objoid = e.oid " "LEFT JOIN pg_catalog.pg_available_extensions()
ae(name, default_version, comment) ON ae.name <http://ae.name> = e.extname
" "AND d.classoid = 'pg_catalog.pg_extension'::pg_catalog.regclass\n",*
```

We may need to handle this by moving the AND condition to the left join for
pg_catalog.pg_description, so that we retrieve only pg_extension entries
from pg_description.
Attaching the patch for the same.

Thoughts?

[1]
https://www.postgresql.org/message-id/CABUevEyTMyXC6OvCWkj+rPnHrfi8_Rw_+DD_jzgFFNPqgf+Oig@mail.gmail.com
--

Thanks & Regards,
Suraj kharage,

enterprisedb.com <https://www.enterprisedb.com/>