plperl: Fix memory leak in hek2cstr
plperl: Fix memory leak in hek2cstr

Backpatch all the way back to 9.1, where it was introduced by commit
50d89d42.

Reported by Sergey Burladyan in #9223
Author: Alex Hunsaker

zeroae/postgres-xl,greenplum-db/gpdb,oberstet/postgres-xl,50wu/gpdb,50wu/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,50wu/gpdb,zeroae/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,yazun/postgres-xl,adam8157/gpdb,greenplum-db/gpdb,techdragon/Postgres-XL,50wu/gpdb,xinzweb/gpdb,ovr/postgres-xl,xinzweb/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,greenplum-db/gpdb,Postgres-XL/Postgres-XL,ashwinstar/gpdb,lisakowen/gpdb,yazun/postgres-xl,lisakowen/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,oberstet/postgres-xl,pavanvd/postgres-xl,oberstet/postgres-xl,Postgres-XL/Postgres-XL,adam8157/gpdb,ovr/postgres-xl,zeroae/postgres-xl,pavanvd/postgres-xl,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,ashwinstar/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,yazun/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,zeroae/postgres-xl,techdragon/Postgres-XL,lisakowen/gpdb,techdragon/Postgres-XL,Postgres-XL/Postgres-XL,ovr/postgres-xl,techdragon/Postgres-XL,ashwinstar/gpdb,greenplum-db/gpdb,adam8157/gpdb,ashwinstar/gpdb,Postgres-XL/Postgres-XL,adam8157/gpdb,lisakowen/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,greenplum-db/gpdb,zeroae/postgres-xl,50wu/gpdb,oberstet/postgres-xl,xinzweb/gpdb,greenplum-db/gpdb,ovr/postgres-xl,yazun/postgres-xl,ovr/postgres-xl,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,Postgres-XL/Postgres-XL,adam8157/gpdb