Fix crash after squelching a RecursiveUnion node.
Fix crash after squelching a RecursiveUnion node.

If a RecursiveUnion is rescanned after squelching, we still need the
tuplestores. So don't free them, just reset them to empty. This showed
up as crashes in the regression suite.

Reviewed-by: Pengzhou Tang <ebfb6fa79a634ed64a59bdd78edfa125b950b4d8@pivotal.io>

lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,ashwinstar/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,adam8157/gpdb,xinzweb/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,50wu/gpdb,greenplum-db/gpdb