Fix handling of non-key columns get_index_column_opclass()

Fix handling of non-key columns get_index_column_opclass()

f2e40380 introduces support of non-key attributes in GiST indexes.  Then if
get_index_column_opclass() is asked by gistproperty() to get an opclass of
non-key column, it returns garbage past oidvector value.  This commit fixes
that by making get_index_column_opclass() return InvalidOid in this case.

Discussion: https://postgr.es/m/20190902231948.GA5343%40alvherre.pgsql
Author: Nikita Glukhov, Alexander Korotkov
Backpatch-through: 12