Properly initialize SortSupport for ORDER BY rechecks in nodeIndexscan.c.

Fix still another bug in commit 35fcb1b3: it failed to fully initialize
the SortSupport states it introduced to allow the executor to re-check
ORDER BY expressions containing distance operators.  That led to a null
pointer dereference if the sortsupport code tried to use ssup_cxt.  The
problem only manifests in narrow cases, explaining the lack of previous
field reports.  It requires a GiST-indexable distance operator that lacks
SortSupport and is on a pass-by-ref data type, which among core+contrib
seems to be only btree_gist's interval opclass; and it requires the scan
to be done as an IndexScan not an IndexOnlyScan, which explains how
btree_gist's regression test didn't catch it.  Per bug #14134 from
Jihyun Yu.

Peter Geoghegan

Report: <20160511154904.2603.43889@wrigleys.postgresql.org>