Further fixes to the pg_get_expr() security fix in back branches.
Further fixes to the pg_get_expr() security fix in back branches.

It now emerges that the JDBC driver expects to be able to use pg_get_expr()
on an output of a sub-SELECT.  So extend the check logic to be able to recurse
into a sub-SELECT to see if the argument is ultimately coming from an
appropriate column.  Per report from Thomas Kellerer.

royc1/gpdb,edespino/gpdb,jmcatamney/gpdb,50wu/gpdb,greenplum-db/gpdb,edespino/gpdb,Quikling/gpdb,50wu/gpdb,ahachete/gpdb,rvs/gpdb,Quikling/gpdb,lintzc/gpdb,cjcjameson/gpdb,lisakowen/gpdb,zaksoup/gpdb,edespino/gpdb,xinzweb/gpdb,Chibin/gpdb,chrishajas/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,CraigHarris/gpdb,ahachete/gpdb,janebeckman/gpdb,yuanzhao/gpdb,CraigHarris/gpdb,0x0FFF/gpdb,royc1/gpdb,adam8157/gpdb,rvs/gpdb,lisakowen/gpdb,lisakowen/gpdb,CraigHarris/gpdb,cjcjameson/gpdb,xinzweb/gpdb,Quikling/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,Chibin/gpdb,janebeckman/gpdb,0x0FFF/gpdb,0x0FFF/gpdb,ahachete/gpdb,rvs/gpdb,cjcjameson/gpdb,adam8157/gpdb,Quikling/gpdb,lisakowen/gpdb,50wu/gpdb,Chibin/gpdb,xuegang/gpdb,ahachete/gpdb,Chibin/gpdb,xinzweb/gpdb,adam8157/gpdb,yuanzhao/gpdb,edespino/gpdb,royc1/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,chrishajas/gpdb,royc1/gpdb,Quikling/gpdb,xuegang/gpdb,kaknikhil/gpdb,greenplum-db/gpdb,Quikling/gpdb,kaknikhil/gpdb,lisakowen/gpdb,chrishajas/gpdb,CraigHarris/gpdb,xuegang/gpdb,lisakowen/gpdb,janebeckman/gpdb,yuanzhao/gpdb,lisakowen/gpdb,edespino/gpdb,rvs/gpdb,cjcjameson/gpdb,cjcjameson/gpdb,royc1/gpdb,xuegang/gpdb,edespino/gpdb,CraigHarris/gpdb,50wu/gpdb,janebeckman/gpdb,adam8157/gpdb,lintzc/gpdb,rvs/gpdb,ashwinstar/gpdb,janebeckman/gpdb,edespino/gpdb,jmcatamney/gpdb,CraigHarris/gpdb,xinzweb/gpdb,50wu/gpdb,jmcatamney/gpdb,lintzc/gpdb,Quikling/gpdb,xinzweb/gpdb,adam8157/gpdb,0x0FFF/gpdb,rvs/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,Chibin/gpdb,janebeckman/gpdb,greenplum-db/gpdb,0x0FFF/gpdb,cjcjameson/gpdb,rvs/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,janebeckman/gpdb,yuanzhao/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,edespino/gpdb,Quikling/gpdb,lintzc/gpdb,greenplum-db/gpdb,chrishajas/gpdb,ahachete/gpdb,edespino/gpdb,xinzweb/gpdb,chrishajas/gpdb,ahachete/gpdb,jmcatamney/gpdb,janebeckman/gpdb,50wu/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,kaknikhil/gpdb,rvs/gpdb,xinzweb/gpdb,jmcatamney/gpdb,xinzweb/gpdb,royc1/gpdb,lintzc/gpdb,yuanzhao/gpdb,lisakowen/gpdb,zaksoup/gpdb,xuegang/gpdb,Chibin/gpdb,CraigHarris/gpdb,janebeckman/gpdb,lintzc/gpdb,Quikling/gpdb,adam8157/gpdb,Chibin/gpdb,ahachete/gpdb,cjcjameson/gpdb,chrishajas/gpdb,ahachete/gpdb,0x0FFF/gpdb,adam8157/gpdb,lintzc/gpdb,zaksoup/gpdb,Quikling/gpdb,edespino/gpdb,zaksoup/gpdb,zaksoup/gpdb,kaknikhil/gpdb,cjcjameson/gpdb,kaknikhil/gpdb,yuanzhao/gpdb,Chibin/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,0x0FFF/gpdb,royc1/gpdb,Chibin/gpdb,adam8157/gpdb,CraigHarris/gpdb,50wu/gpdb,xuegang/gpdb,kaknikhil/gpdb,xuegang/gpdb,Chibin/gpdb,greenplum-db/gpdb,kaknikhil/gpdb,CraigHarris/gpdb,chrishajas/gpdb,lintzc/gpdb,chrishajas/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,50wu/gpdb,xuegang/gpdb,royc1/gpdb,zaksoup/gpdb,xuegang/gpdb,ashwinstar/gpdb,cjcjameson/gpdb,rvs/gpdb,janebeckman/gpdb,rvs/gpdb,cjcjameson/gpdb,0x0FFF/gpdb,zaksoup/gpdb,lintzc/gpdb,zaksoup/gpdb,greenplum-db/gpdb