pgcrypto: Code cleanup for decrypt_internal.
pgcrypto: Code cleanup for decrypt_internal.

Remove some unnecessary null-tests, and replace a goto-label construct
with an "if" block.

Michael Paquier, reviewed by me.

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