Add sanity checks, to check that a subplan is initialized.
Add sanity checks, to check that a subplan is initialized.

I've run into NULL-pointer references here several times during
development, when I have screwed up the executor initialization so that
some subplans were not initialized in the correct slices. It's clearly a
bug if that happens, but I think a sanity check is in order.

Reviewed-by: Soumyadeep Chakraborty <ec19037653195c8fe6913bfa51ee2dd46d490579@pivotal.io>

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