btree_gin: Incorrect leftmost interval value

In contrib/btree_gin, leftmostvalue_interval() does this:

leftmostvalue_interval(void)
{
    Interval   *v = palloc(sizeof(Interval));

    v->time = DT_NOBEGIN;
    v->day = 0;
    v->month = 0;
    return IntervalPGetDatum(v);
}

which is a long way short of the minimum possible interval value.

As a result, a < or <= query using a GIN index on an interval column
may miss values. For example:

CREATE EXTENSION btree_gin;
CREATE TABLE foo (a interval);
INSERT INTO foo VALUES ('-1000000 years');
CREATE INDEX foo_idx ON foo USING gin (a);

SET enable_seqscan = off;
SELECT * FROM foo WHERE a < '1 year';
 a
---
(0 rows)

Attached is a patch fixing this by setting all the fields to their
minimum values, which is guaranteed to be less than any other
interval.

Note that this doesn't affect the contents of the index itself, so
reindexing is not necessary.

Regards,
Dean