Check for valid columns in adodb postgres64 driver MetaIndexes fn

[Kristina Hoeppner]

Indexes on functions of columns have a 0 in the indkey column of pg_catalog.pg_index, which doesn't refer to a column. Don't return any columns for the index, rather than returning an empty column name.

[Kristina Hoeppner (kris-hoeppner)]

https://reviews.mahara.org/#/c/7528/