pgsql: If an index depends on no columns of its table, give it a

Log Message:
-----------
If an index depends on no columns of its table, give it a dependency on the
whole table instead, to ensure that it goes away when the table is dropped.
Per bug #3723 from Sam Mason.

Backpatch as far as 7.4; AFAICT 7.3 does not have the issue, because it doesn't
have general-purpose expression indexes and so there must be at least one
column referenced by an index.

Modified Files:
--------------
    pgsql/src/backend/catalog:
        index.c (r1.286 -> r1.287)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c?r1=1.286&r2=1.287)