Do not allow *timestamp to be passed as NULL
Do not allow *timestamp to be passed as NULL

The code had bugs that would cause crashes if NULL was passed as that
argument (originally intended to mean not to bother returning its
value), and after inspection it turns out that nothing seems interested
in the case that *ts is NULL anyway.  Therefore, remove the partial
checks intended to support that case.

Author: Michael Paquier
though I didn't include a proposed Assert.

Backpatch to 9.5.

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