Fix assert to allow zero.  OK'ed by Bryan.
Fix assert to allow zero.  OK'ed by Bryan.

zeroae/postgres-xl,snaga/postgres-xl,xinzweb/gpdb,postmind-net/postgres-xl,Chibin/gpdb,techdragon/Postgres-XL,Quikling/gpdb,foyzur/gpdb,yuanzhao/gpdb,pavanvd/postgres-xl,zeroae/postgres-xl,lisakowen/gpdb,adam8157/gpdb,CraigHarris/gpdb,adam8157/gpdb,Chibin/gpdb,zaksoup/gpdb,tangp3/gpdb,janebeckman/gpdb,ovr/postgres-xl,Quikling/gpdb,rvs/gpdb,randomtask1155/gpdb,randomtask1155/gpdb,cjcjameson/gpdb,rubikloud/gpdb,foyzur/gpdb,yazun/postgres-xl,lintzc/gpdb,greenplum-db/gpdb,edespino/gpdb,atris/gpdb,postmind-net/postgres-xl,xuegang/gpdb,Quikling/gpdb,janebeckman/gpdb,ahachete/gpdb,chrishajas/gpdb,pavanvd/postgres-xl,pavanvd/postgres-xl,Postgres-XL/Postgres-XL,0x0FFF/gpdb,50wu/gpdb,chrishajas/gpdb,Postgres-XL/Postgres-XL,adam8157/gpdb,chrishajas/gpdb,kmjungersen/PostgresXL,yuanzhao/gpdb,royc1/gpdb,lisakowen/gpdb,Chibin/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,foyzur/gpdb,Postgres-XL/Postgres-XL,xuegang/gpdb,Postgres-XL/Postgres-XL,tangp3/gpdb,cjcjameson/gpdb,ahachete/gpdb,rubikloud/gpdb,Quikling/gpdb,xinzweb/gpdb,lisakowen/gpdb,janebeckman/gpdb,ahachete/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,xuegang/gpdb,rvs/gpdb,rvs/gpdb,Quikling/gpdb,CraigHarris/gpdb,yuanzhao/gpdb,tangp3/gpdb,rubikloud/gpdb,janebeckman/gpdb,ovr/postgres-xl,randomtask1155/gpdb,lpetrov-pivotal/gpdb,yazun/postgres-xl,rvs/gpdb,ahachete/gpdb,greenplum-db/gpdb,50wu/gpdb,tpostgres-projects/tPostgres,atris/gpdb,kmjungersen/PostgresXL,atris/gpdb,yuanzhao/gpdb,postmind-net/postgres-xl,Chibin/gpdb,jmcatamney/gpdb,tpostgres-projects/tPostgres,0x0FFF/gpdb,arcivanov/postgres-xl,atris/gpdb,oberstet/postgres-xl,yuanzhao/gpdb,50wu/gpdb,kaknikhil/gpdb,edespino/gpdb,snaga/postgres-xl,edespino/gpdb,Chibin/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lintzc/gpdb,tangp3/gpdb,techdragon/Postgres-XL,lpetrov-pivotal/gpdb,randomtask1155/gpdb,atris/gpdb,edespino/gpdb,Quikling/gpdb,royc1/gpdb,xuegang/gpdb,postmind-net/postgres-xl,0x0FFF/gpdb,foyzur/gpdb,zeroae/postgres-xl,xinzweb/gpdb,foyzur/gpdb,lisakowen/gpdb,chrishajas/gpdb,0x0FFF/gpdb,jmcatamney/gpdb,tpostgres-projects/tPostgres,lisakowen/gpdb,ahachete/gpdb,adam8157/gpdb,edespino/gpdb,ashwinstar/gpdb,ahachete/gpdb,Postgres-XL/Postgres-XL,kaknikhil/gpdb,yazun/postgres-xl,ashwinstar/gpdb,arcivanov/postgres-xl,CraigHarris/gpdb,lpetrov-pivotal/gpdb,randomtask1155/gpdb,janebeckman/gpdb,yuanzhao/gpdb,kaknikhil/gpdb,CraigHarris/gpdb,arcivanov/postgres-xl,0x0FFF/gpdb,kaknikhil/gpdb,rvs/gpdb,kmjungersen/PostgresXL,50wu/gpdb,chrishajas/gpdb,snaga/postgres-xl,lintzc/gpdb,atris/gpdb,greenplum-db/gpdb,janebeckman/gpdb,janebeckman/gpdb,Chibin/gpdb,edespino/gpdb,Quikling/gpdb,adam8157/gpdb,tangp3/gpdb,CraigHarris/gpdb,atris/gpdb,Chibin/gpdb,0x0FFF/gpdb,rvs/gpdb,zaksoup/gpdb,ovr/postgres-xl,oberstet/postgres-xl,cjcjameson/gpdb,xuegang/gpdb,ahachete/gpdb,kmjungersen/PostgresXL,rubikloud/gpdb,foyzur/gpdb,jmcatamney/gpdb,zaksoup/gpdb,lintzc/gpdb,tpostgres-projects/tPostgres,zaksoup/gpdb,jmcatamney/gpdb,zaksoup/gpdb,ashwinstar/gpdb,yuanzhao/gpdb,xuegang/gpdb,xuegang/gpdb,kmjungersen/PostgresXL,foyzur/gpdb,cjcjameson/gpdb,yazun/postgres-xl,CraigHarris/gpdb,oberstet/postgres-xl,janebeckman/gpdb,CraigHarris/gpdb,rubikloud/gpdb,Quikling/gpdb,snaga/postgres-xl,lintzc/gpdb,royc1/gpdb,royc1/gpdb,lpetrov-pivotal/gpdb,snaga/postgres-xl,lintzc/gpdb,edespino/gpdb,adam8157/gpdb,arcivanov/postgres-xl,techdragon/Postgres-XL,lpetrov-pivotal/gpdb,cjcjameson/gpdb,tpostgres-projects/tPostgres,rubikloud/gpdb,kaknikhil/gpdb,lintzc/gpdb,royc1/gpdb,chrishajas/gpdb,rubikloud/gpdb,zeroae/postgres-xl,50wu/gpdb,ovr/postgres-xl,zaksoup/gpdb,xuegang/gpdb,yuanzhao/gpdb,kaknikhil/gpdb,rubikloud/gpdb,randomtask1155/gpdb,kaknikhil/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,kaknikhil/gpdb,randomtask1155/gpdb,xinzweb/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,CraigHarris/gpdb,tangp3/gpdb,rvs/gpdb,tangp3/gpdb,edespino/gpdb,Quikling/gpdb,techdragon/Postgres-XL,xinzweb/gpdb,zeroae/postgres-xl,yazun/postgres-xl,lpetrov-pivotal/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,atris/gpdb,royc1/gpdb,ashwinstar/gpdb,edespino/gpdb,cjcjameson/gpdb,pavanvd/postgres-xl,janebeckman/gpdb,royc1/gpdb,ashwinstar/gpdb,rvs/gpdb,ahachete/gpdb,foyzur/gpdb,adam8157/gpdb,lintzc/gpdb,jmcatamney/gpdb,chrishajas/gpdb,jmcatamney/gpdb,rvs/gpdb,CraigHarris/gpdb,0x0FFF/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,0x0FFF/gpdb,cjcjameson/gpdb,Chibin/gpdb,lintzc/gpdb,tangp3/gpdb,Chibin/gpdb,ashwinstar/gpdb,kaknikhil/gpdb,Quikling/gpdb,zaksoup/gpdb,adam8157/gpdb,xuegang/gpdb,cjcjameson/gpdb,ovr/postgres-xl,lpetrov-pivotal/gpdb,rvs/gpdb,randomtask1155/gpdb,zaksoup/gpdb,greenplum-db/gpdb,lpetrov-pivotal/gpdb,lisakowen/gpdb,postmind-net/postgres-xl,50wu/gpdb,cjcjameson/gpdb,kaknikhil/gpdb,oberstet/postgres-xl,Chibin/gpdb,arcivanov/postgres-xl,edespino/gpdb,arcivanov/postgres-xl,cjcjameson/gpdb,janebeckman/gpdb,chrishajas/gpdb,techdragon/Postgres-XL,yuanzhao/gpdb,greenplum-db/gpdb,royc1/gpdb