Eliminate some no-longer-needed workarounds for palloc's old behavior of rejecting palloc(0).  Also, tweak like_selectivity() to avoid assuming the presented pattern is nonempty; although that assumption is valid, it doesn't really help much, and the new coding is more correct anyway since it properly handles redundant wildcards.  In combination these changes should eliminate a Coverity warning noted by Martijn.
Eliminate some no-longer-needed workarounds for palloc's old behavior
of rejecting palloc(0).  Also, tweak like_selectivity() to avoid assuming
the presented pattern is nonempty; although that assumption is valid,
it doesn't really help much, and the new coding is more correct anyway
since it properly handles redundant wildcards.  In combination these
changes should eliminate a Coverity warning noted by Martijn.

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