Fix bug in the tsvector stats collection function, which caused a crash if the sample contains just a one tsvector, containing only one lexeme.
Fix bug in the tsvector stats collection function, which caused a crash if
the sample contains just a one tsvector, containing only one lexeme.

oberstet/postgres-xl,Chibin/gpdb,zeroae/postgres-xl,ovr/postgres-xl,kmjungersen/PostgresXL,Postgres-XL/Postgres-XL,tpostgres-projects/tPostgres,kmjungersen/PostgresXL,xinzweb/gpdb,oberstet/postgres-xl,adam8157/gpdb,yuanzhao/gpdb,arcivanov/postgres-xl,Chibin/gpdb,techdragon/Postgres-XL,snaga/postgres-xl,jmcatamney/gpdb,postmind-net/postgres-xl,edespino/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,postmind-net/postgres-xl,greenplum-db/gpdb,zeroae/postgres-xl,yuanzhao/gpdb,xinzweb/gpdb,oberstet/postgres-xl,pavanvd/postgres-xl,Chibin/gpdb,edespino/gpdb,oberstet/postgres-xl,yazun/postgres-xl,adam8157/gpdb,kmjungersen/PostgresXL,pavanvd/postgres-xl,Postgres-XL/Postgres-XL,yuanzhao/gpdb,edespino/gpdb,yazun/postgres-xl,jmcatamney/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,techdragon/Postgres-XL,ovr/postgres-xl,arcivanov/postgres-xl,edespino/gpdb,ovr/postgres-xl,ashwinstar/gpdb,yuanzhao/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,pavanvd/postgres-xl,postmind-net/postgres-xl,yazun/postgres-xl,Chibin/gpdb,Chibin/gpdb,edespino/gpdb,xinzweb/gpdb,greenplum-db/gpdb,snaga/postgres-xl,lisakowen/gpdb,zeroae/postgres-xl,lisakowen/gpdb,xinzweb/gpdb,ovr/postgres-xl,edespino/gpdb,edespino/gpdb,Chibin/gpdb,yuanzhao/gpdb,lisakowen/gpdb,Chibin/gpdb,zeroae/postgres-xl,techdragon/Postgres-XL,jmcatamney/gpdb,lisakowen/gpdb,postmind-net/postgres-xl,snaga/postgres-xl,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,Postgres-XL/Postgres-XL,yazun/postgres-xl,adam8157/gpdb,Chibin/gpdb,yuanzhao/gpdb,Chibin/gpdb,jmcatamney/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,snaga/postgres-xl,ashwinstar/gpdb,edespino/gpdb,zeroae/postgres-xl,adam8157/gpdb,arcivanov/postgres-xl,tpostgres-projects/tPostgres,jmcatamney/gpdb,50wu/gpdb,adam8157/gpdb,edespino/gpdb,ashwinstar/gpdb,techdragon/Postgres-XL,adam8157/gpdb,ashwinstar/gpdb,50wu/gpdb,xinzweb/gpdb,ovr/postgres-xl,tpostgres-projects/tPostgres,tpostgres-projects/tPostgres,50wu/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,50wu/gpdb,ashwinstar/gpdb,snaga/postgres-xl,postmind-net/postgres-xl,lisakowen/gpdb,pavanvd/postgres-xl,ashwinstar/gpdb,adam8157/gpdb,tpostgres-projects/tPostgres,techdragon/Postgres-XL,Chibin/gpdb,ashwinstar/gpdb,kmjungersen/PostgresXL,xinzweb/gpdb,arcivanov/postgres-xl,arcivanov/postgres-xl,kmjungersen/PostgresXL,arcivanov/postgres-xl,Postgres-XL/Postgres-XL,50wu/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,lisakowen/gpdb,yazun/postgres-xl,oberstet/postgres-xl,lisakowen/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,edespino/gpdb,50wu/gpdb